I had the same problem on mine after I put in the 2.5" lift. It only happened on long trips at 65mph+ and when I let off on the gas. Toyota thought it was the diff and replaced the pumpkin (on warranty). It turned out that it was due to the changed pinion angles; a set of upper Slee rear control arm extensions should alleviate the problem. I've not had the problem again since I installed them.