This is a useful example of how to drag rows from grids onto trees:
https://examples2.ext.net/#/DragDrop/Grid/Grid_to_Tree/

However, it removes the record from the grid, because a tree node representing that record is created. This is great for many scenarios.

However, I had a different scenario:
- Drag row(s) onto a tree node
- Prevent tree nodes from being dragged to tree
- Get those row ids and call some AJAX/DirectMethod to process those
- NOT remove the records from the grid (by default the records are removed)

To prevent removing records from the grid when they have been dropped, the trick was to empty out the records passed into the beforedrop method.

Thanks to this forum post at Sencha for the tips:
http://www.sencha.com/forum/showthre...ng-a-tree-node

So a final working solution:

<%@ Page Language="C#" %>
 
<script runat="server">
    private object[] TestData
    {
        get
        {
            DateTime now = DateTime.Now;
 
            return new object[]
            {
                new object[] { "3m Co", 71.72, 0.02, 0.03, now },
                new object[] { "Alcoa Inc", 29.01, 0.42, 1.47, now },
                new object[] { "Altria Group Inc", 83.81, 0.28, 0.34, now },
                new object[] { "American Express Company", 52.55, 0.01, 0.02, now },
                new object[] { "American International Group, Inc.", 64.13, 0.31, 0.49, now },
                new object[] { "AT&T Inc.", 31.61, -0.48, -1.54, now },
                new object[] { "Boeing Co.", 75.43, 0.53, 0.71, now },
                new object[] { "Caterpillar Inc.", 67.27, 0.92, 1.39, now },
                new object[] { "Citigroup, Inc.", 49.37, 0.02, 0.04, now },
                new object[] { "E.I. du Pont de Nemours and Company", 40.48, 0.51, 1.28, now },
                new object[] { "Exxon Mobil Corp", 68.1, -0.43, -0.64, now },
                new object[] { "General Electric Company", 34.14, -0.08, -0.23, now },
                new object[] { "General Motors Corporation", 30.27, 1.09, 3.74, now },
                new object[] { "Hewlett-Packard Co.", 36.53, -0.03, -0.08, now },
                new object[] { "Honeywell Intl Inc", 38.77, 0.05, 0.13, now },
                new object[] { "Intel Corporation", 19.88, 0.31, 1.58, now },
                new object[] { "International Business Machines", 81.41, 0.44, 0.54, now },
                new object[] { "Johnson & Johnson", 64.72, 0.06, 0.09, now },
                new object[] { "JP Morgan & Chase & Co", 45.73, 0.07, 0.15, now },
                new object[] { "McDonald\"s Corporation", 36.76, 0.86, 2.40, now },
                new object[] { "Merck & Co., Inc.", 40.96, 0.41, 1.01, now },
                new object[] { "Microsoft Corporation", 25.84, 0.14, 0.54, now },
                new object[] { "Pfizer Inc", 27.96, 0.4, 1.45, now },
                new object[] { "The Coca-Cola Company", 45.07, 0.26, 0.58, now },
                new object[] { "The Home Depot, Inc.", 34.64, 0.35, 1.02, now },
                new object[] { "The Procter & Gamble Company", 61.91, 0.01, 0.02, now },
                new object[] { "United Technologies Corporation", 63.26, 0.55, 0.88, now },
                new object[] { "Verizon Communications", 35.57, 0.39, 1.11, now },
                new object[] { "Wal-Mart Stores, Inc.", 45.45, 0.73, 1.63, now }
            };
        }
    }
    
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!X.IsAjaxRequest)
        {
            this.Store1.DataSource = this.TestData;
            this.Store1.DataBind(); 
        }
    }
</script>
 
<!DOCTYPE html>
 
<html>
<head runat="server">
    <title>Drag and Drop from GridPanel to TreePanel - Ext.NET Examples</title>
 
    <script>
        var beforedrop = function(node, data, overModel, dropPos, opts) {
            this.droppedRecords = data.records;
            data.records = [];
        };
 
        var drop = function(node, data, overModel, dropPos, opts) {
            var str = '';
 
            Ext.iterate(this.droppedRecords, function (record) {
                str += record.get('title') + ' (id = ' + record.get('id') + ') dropped on ' + overModel.get('text') + '\n';
            });
            console.log(str);
                    
            delete this.droppedRecords;
        }
    </script>
</head>
<body>
    <form runat="server">
        <ext:ResourceManager runat="server" />
        
        <ext:Panel 
            runat="server"
            Width="700" 
            Height="400"
            Layout="BorderLayout">
            <Items>
                <ext:GridPanel 
                    runat="server"
                    Region="Center"
                    Title="Grid" 
                    MultiSelect="true"
                    MarginSpec="5 0 5 5">
                    <Store>
                        <ext:Store 
                            ID="Store1" 
                            runat="server">
                            <Model>
                                <ext:Model runat="server">
                                    <Fields>
                                        <ext:ModelField Name="company" />
                                        <ext:ModelField Name="price" Type="Float" />
                                        <ext:ModelField Name="change" Type="Float" />
                                        <ext:ModelField Name="pctChange" Type="Float" />
                                        <ext:ModelField Name="lastChange" Type="Date"/>
                                    </Fields>
                                </ext:Model>
                            </Model>
                        </ext:Store>                    
                    </Store>
                    <ColumnModel runat="server">
                        <Columns>
                            <ext:Column 
                                runat="server" 
                                Text="Company" 
                                Sortable="true" 
                                DataIndex="company"
                                Flex="1" 
                                />
                            <ext:Column 
                                runat="server" 
                                Text="Price" 
                                Width="75" 
                                Sortable="true" 
                                DataIndex="price">
                                <Renderer Format="UsMoney" />
                            </ext:Column>
                        </Columns>
                    </ColumnModel>                    
                    <View>
                        <ext:GridView runat="server">
                            <Plugins>
                                <ext:GridDragDrop runat="server" DragGroup="grid2tree" />
                            </Plugins>
                        </ext:GridView>
                    </View>
                </ext:GridPanel>
                    
                <ext:TreePanel 
                    runat="server" 
                    Region="East"                    
                    Width="300"
                    Title="Tree"
                    AutoScroll="true"                    
                    MarginSpec="5 5 5 0"
                    Split="true">
                    <Root>
                        <ext:Node Text="Root" Expanded="true">                        
                            <Children>
                                <ext:Node Text="Folder 1">
                                    <Children>
                                        <ext:Node Text="Subleaf 1" Qtip="Subleaf 1 Quick Tip" Leaf="true" />
                                    </Children>
                                </ext:Node>
                                        
                                <ext:Node Text="Folder 2">
                                    <Children>
                                        <ext:Node Text="Subleaf 2" Leaf="true" />
                                    </Children>
                                </ext:Node>
                                        
                                <ext:Node Text="Leaf 1" Leaf="true" />
                            </Children>
                        </ext:Node>
                    </Root>
                    <View>
                        <ext:TreeView runat="server">
                            <Plugins>
                                <ext:TreeViewDragDrop runat="server" DropGroup="grid2tree" EnableDrag="false" />
                            </Plugins>
                            <Listeners>
                                <BeforeDrop Fn="beforedrop" />
                                <Drop Fn="drop" />
                            </Listeners>
                        </ext:TreeView>
                    </View>              
                </ext:TreePanel>
            </Items>
        </ext:Panel> 
    </form>
</body>
</html>
In the above example, I am writing out some basic info to the browser console (as per the example from the Sencha forums). It is here where the data can be passed to the server via ajax if needed.

Hope that helps someone!