Indeed, the RadTreeNodes do not have client IDs. However, you can either assign such on the server or create the tooltip dynamically on the client. For such cases we have prepared the following demo which shows both the approaches:
If you want to tooltipify only particular nodes, you should add the only their ClientIDs (which you programmatically set) to the TargetControls collection or create dynamically tooltips only when they are hovered - in this manner the other nodes will not get tooltipified.
Note, also, that if you have ajaxified the sorting, you should make sure that you also update teh tooltip manager with AJAX when sorting.
Please, examine the code and use it as a start point for your implementation.
On a side note, when you install RadControls, our online examples projects are automatically installed on your computer and you can not only examine their full source code, but also extend them further in order to meet your requirements.
You can open the demos from here:
Start\Programs\Telerik\RadControls for ASPNET AJAX\Open Sample WebSite In VisualStudio
The actual location on your hard disk is the following one:
C:\Program Files\Telerik\RadControls for ASPNET AJAX \Live Demos
I hope that this information is helpful, let me know if you have additional questions or you need further assistance.
the Telerik team