Having just replaced the diff in mine I can almost be sure you have the same issue, broken thrust balls. If you remove the diff adjusting screw and the thrust bearing you can easily check them to see if they are all there or some have been crushed. The easy way out for me was to buy the complete B-Fast pro kit which is proving to be a much stronger bit of gear
http://www.bfastrc.com/tqracing.html
Here's the link for it, otherwise you could just get new thrust bearings and build the diff really carefully next time but meh, for the small price difference you might as well get the Bfast kit and be done with it