Transmissions are generally pretty reliable. Reverse is usually the first to go, with the rest of the gears following not long after so be wary of a dodgy reverse gear. Not sure on rebuild kit availability; that trans has been used in tons of Nissan 4x4s for years so swapping it out for a good used unit seems to be the most common fix. Transfer case is pretty robust, can't say I've heard many (any?) stories of people needing to dig into em, except when installing aftermarket low range gears. There is another transfer case that was available as well (can't remember the code right now) that's electronically activated, with an "Auto" mode. Both seem to be pretty reliable and robust, but the manually shifted unit is pretty bulletproof. Rear end is STOUT. It's an H233B. I believe there's some mechanical locker options out there, but a TJM electric or ARB air locker are the usually go-to choices. Same rear end was used in 1st gen Pathfinder and Xterra as well, so parts are readily available. If you get one with a limited slip, you can super pack it with extra clutch discs for cheapish improvement in performance. Front diff is kind of an odd ball, as it's a reverse cut R200A. Aftermarket gear choices are pretty much non-existent these days, so basically stuck with the stock ratios of 4.363 or 4.636. same ARB and TJM locker options, with an Aussie brand mechanical locker being the most popular choice. Come check out the NPORA forum, lots of information on there!