I can't remember how many times I've bonked and dragged my unprotected diff over rocks and such and it hasn't hurt it a bit. Yeah, I had to take a dremel with a carbide bit to re-square the recess in the drain plug when I changed the fluid a few weeks ago, but it put it right back in afterwards. All the diff skid would do for me is reduce clearance, catch mud, and make changing the diff fluid more of a chore. Now, if it had a nice nose that went out and protected the u-joint and yoke like some I've seen, that might be worth it, but the housing and third member can take plenty of abuse in my experience.
Brent