I think one of the big concerns with coating the undercarriage in something as thick as bed liner is that it'll trap moisture. Over time if the bed liner receives chips/cracks that let moisture in and then not out, you could have a rust nightmare on your hands. Often it can be better to have your normal painted metal so at least it can breath. I've never lived in a rusty area, but people say really good things about annual coatings of oily rust preventatives.

First off, best to start a thread for this, the status update thing is kinda awkward to read and use.

I've seen a few threads on bedlining the sides, not sure about underneath, but yeah, if it comes unstuck it'll make a lovely moisture trap. A friend of mine bedlined the sides of his S10 Blazer and it's worked out pretty well for him. Hides some questionable bodywork, too.

Ratcheting or air lockers are available for these. IIRC the ratcheting ones install in the open diff carrier in place of the spider gears, the air lockers replace the carrier entirely. The front's an R200A and the rear is a 33-spline H233B, if that helps. There was also a factory limited slip that can be rebuilt for better lockup, or IIRC you can use the 3rd member from a first-year Xterra (higher breakaway torque than the R50s got). The later years of R50 really got hosed with their LSDs, they have much lower breakaway torque than earlier models. Hawairish has done some great writeups on messing with axles.

Echoing Slartbarfast's suggestion to start a thread if you haven't already...it's the best way to get some visibility.

Regarding traction devices, I've run open/open (front/rear), lunchbox/re-packed LSD, and am currently air/air locked.  I've also done a fair amount of research on available e-lockers.  Any one of those devices by itself is a great improvement over an open/open setup.  I do enjoy the selectability of the air lockers, and have not encountered any adverse effects, but I personally hate doing pneumatic work now because of them (and they also require as much electrical work as e-lockers).  Given the chance to do it again, I'd probably go the e-locker route...cost-wise, it pretty much nets out, but the advantage of an air setup is that it often adds tire-inflation capabilities depending on the chosen compressor.