I wouldn't even say there is a real problem. The trouble Lighforce is having with his rear diff case is due to the track he runs on. It's really rough and he comes in and out of major bumps on throttle. I recently had a rear diff come apart but only due to a major crash on the rear end. I've been running my car since for 15 months so it took a while to bust.
Parts broken in 15 months on the XB:
1) 2x stock 19mm wheel bearings. I replaced em all with VXB's a loooong time ago and didn't have a problem up till I upgraded to the 7075 RC knuckles.
2) 1x CVD. I ran right into a HPI Savage at WOT.
3) 1x XB body.
4) A set of stock tires.
5) 2x front upper turnbuckes (dam cartwheels)
All in all I really haven't broken jack compared to what I wouldn't broken on my HPI, FG, or MCD. And yes, I own/owned multiples of all of those cars.