Your car is great! You just need a few things to make the diff stronger. When i was tightening my friend's diff, i actually broke the screw! I suggest replacing the screw, thrust bearing, and t-nut with associated parts.
Bolt/nut:
http://www2.towerhobbies.com/cgi-bin...?&I=LX2869&P=7
Thrust bearing:
http://www2.towerhobbies.com/cgi-bin...?&I=LX2885&P=7
The thrust bearing looks funny, but it actually works better.