stripping sun gears inside metal stampede diff...

I have an old style stampede, and I have a novak rooster, 12t motor, running gearing 12/90(i think), and aluminum idler and aluminum diff...everything works well, but I keep stripping the sun gears(the gears inside the diff), how can I prevent this?????Its happened four times now, and im getting tired of replacing the gears...I can destroy them with just normal driving(donuts, jumps, etc.), no crazy reverse forwards or anything like that. How do the people running the mamba max motor/esc setups keep their stuff from stripping? ive never heard this complaint(except for rare instances). I might just replace the gears but put silly putty in there so the gears are not used. How well does a pede work with a locked diff?