something seriously wrong there then,you either have your alloy plastic diff casing worn and that diff gear then is slightly slipping causing bad gear mesh, or your engine is slightly moving around causing more of the same,there is no way that a gear that strong can break like that unless somthing is seriously amiss.
do you bash a lot?
what layshaft pinion do you use?
I think it broke as Brad came off a jump under power, I've broken a ball drive axle the same way.