If you are breaking part GX12, you need to do the 'diff cup mod' that is posted here. It will fix your problems. I also broke an unmodded diff cup simply running through grass and doing donuts in dirt. They suck and are really weak. Mod them and you will have no more problems. Here is a link to the mod that I found.

http://www.rcuniverse.com/forum/m_3804026/tm.htm

There is a really good detailed description here that shows two different ways of doing the mod. The compression fittings method is easier (IMO):

http://www.rcuniverse.com/forum/m_17...tm.htm#1771683

Good luck and I hope this helps.