Make sure the diff is in the right way. That reminds me, I need to check mine to make sure I installed mine correctly, haha. Is this on an E-Revo? I would say take it apart again and take some pictures of the whole assembly process for us to take a look at.
I had a nitro revo for 2 years before, and nothing have probles with this , but now, i've buy a used revo (a piece of scrap) and order for $300 of new parts, including frame, bulkhead, aluminium diff cup, pushrod and turnbukkles, and convert it to electric.
But this rear diff make me sleep at 2:00AM for past three nights.