Looks like you have some cutting to do. I'd cut as close to the bumper as you can and mount some OEM style brackets to the frame ends. Then add a HD rear cross member, and you should be good to go with whatever bolt-on bumper you want.
I like to pop the port plugs and drain as much assembly fluid as possible, then fill the box with new fluid as much as possible (takes a little while to let gravity do it's thing) before mounting or attaching hoses. Once installed I jack the front wheels off the ground and cycle the steering...
Probably the lockouts are the weakest point (intentionally so). Chromoly shafts are a popular upgrade along with a 30 spline carrier. Not much you can do about the ring/pinion gear.
At some point it probably becomes cheaper/better to get a D44 (or D60).
Bumpers tend to bump stuff (or get stepped on). My concern would be road gravel kicked up by traffic (or my own tires) as much as scrapeing on boulders. Cutting corners because I'm in a hurry usually seems to bite me in the behind eventually. YMMV.
You can BRIEFLY ground the wire to see if it moves/pegs the gauge (indicating the sender is probably the issue... either shot or maybe too much teflon tape preventing thread ground), or if it stays at zero (indicating the gauge is shot, or the wire is broken).
All 4 one or All 4 Fun? If the latter, I've been to several in the past. It's a blast, but there are quite a few people, so you need to have some patience at times. This year may be interesting since we had such a heavy snow season.