Personally I would go:
Rock Rails > T-Case > Gas Tank > Diff
I've rarely hit my gas tank on anything, and nothing my stock skid couldn't handle, though I whaled on my rocker rails on every run I have ever been on, and managed to bend a TnT Custom belly/T-Case skid. I also bent both my stock Diff covers so replaced them with cranes.
Basically put some rock rails on it, wheel it, and replace what ever stock stuff you mess up with beefy aftermarket stuff. Rinse and repeat until you chop it all up and re-build it out of tub (see my advance fab thread)