RC Mania: Have a closer look at the smaller crown gears that go onto the main drive shaft, particularly the rears. Do they look like they are spiraling towards one direction? If so that means that they gears are not properly meshing. Try using the 10mm shims on the diff gear to mesh it closer with the crown. You can also check to see if it's those gears by having a friend hold the front wheels while you hold both rear tires and try to spin them. They shouldn't budge...or the will start clicking/giving way/stripping with considerable force.
I hope this helps.