I believe what you want to do is to place a RadTransitionControl in your RadTabItem, and then swap out the 'Content' of that control.
<telerikNavigation:RadTabItem Header="My Tab">
When you change the Content property of the RadTransitionControl, it executes the transition, and we currently have 7 transitions to choose from.
You may want to check out a video I put together recently that talks about different ways to set the Transition property of the RadTransitionControl here
You could also check out the RadTransitionControl on our Silverlight demo site