I use this, it protects very well and stiffen up the rear, real good quality and strong, i don't regret putting it on my truck. It's made in Sweden and can be found at http://www.mamut.com/Luthagens_RC_hobby/subdet369.htm
This will also keep the center of gravity low compared to raising the whole thing 10mm, not much difference but it's there.

With this you only have to cut out a little bigger hole, then bolt this sucker on and it takes care of the rockproblem.. letting them pass trough, so no big risk to damage the diffcover with rock get jammed between, and it's not very expensive either. I recommend it [sm=thumbup.gif]