Maybe you have hearded about the slipper problem...
I think 90% of diff problems come from this problem. Let me explain...
First, I built my differentials just like in the manual, and after 3 qualifications, my car was crying, really, making a strange noise when it attacked the jumps!
So I disassembled my diffs and discovered that the black grease came in contact with the main plates, crap!
Then I re-built them with a TOP thrustbearing and a few black grease... They seemed perfect, the car was easy, but the differentials went crunchy in the afternoon...
I disassembled my diffs again and discover huge marks on the plates, just as the diff has slipped a lot!!! Very strange because I setup the transmission very precisely (slipper + diffs) many times while the race and it seemed ok...
I've discussed a lot with other Tamiya drivers and it seems that the motor heat is making the center cell very hot, and the slipper is tightening itself while the run!!!
So be careful, if you set your slipper, think about the fact that it will be tighter while the run and it could cause damage to your ball differentials...