problems with the rear dif

[suicideneil]

Sounds like the pinion and ring gear arent meshing very well, or the stock gears are a bit soft (non-hardened metal?)

I would try re-shimming the pinion gear to the ring gear (put some shims between the head of the pinion and the inner bearing), check there are no bad bearings alloying the diff to wiggle around too much, and look to see if there are optional harder gears, or an alloy diff case/cup (I have no clue on the design of the mbx, have a look in the buggy/truggy sub-forum for more ideas).