I drive a 94 D21, 4WD, SE, V6, 5spd, king cab. Great trucks overall.
The engines, whether I4 or V6 run forever. Mine has over 350k on it and still runs great. I'm sure it's not putting out the factory claimed hp and torque, but it still scoots when I need it to. The power difference between the 4 and 6 isn't really much, but the 6 gives better low end torque and pulls harder but the 4 isn't a slouch.
Change the oil in the diff, trans, and x-case. Make sure you use the oils that Nissan calls for. The wrong oil will eat up synchros and pooch the LSD if you have one. Royal Purple and Redline are both good choices. The manual transmissions on these things cannot be over filled. There are a couple of bearings that don't get oiled well when filled to the drain plug. A lot of guys will jack up the driver's side when filling the trans to add a little extra. I took the shifter out and dumped in an extra quart and a half or so (I'm estimating--it's was what was left in the jug). Nissan trannies aren't overfilled until they pump oil out the top. If your tranny whines in neutral and/or most gears it's probably a bearing on the secondary shaft. Very common problem. Mine's been whining for almost 200k miles. It's easier to get a low mileage junkyard trans and swap it than rebuild, so that's my plan this summer. Throwout bearings sometimes get noisy as well. It'll sound like someone flicked a moose's lovespuds with a towel when the clutch engages.
Carrier bearings on the rear drive shaft can be a problem. If your truck doesn't have one count yourself lucky. If the bearing itself fails you can pop the case apart and just replace the bearing. It's about $120 cheaper that way. The bushings go to pot on them sometimes as well. Get a one-piece shaft from a 99-04 Frontier (same x-case, rear end, and wheelbase). It will fit with no mods. That also eliminates a u-joint, so one less thing to maintain or fix.
The tension arms (trailing arms, radius arms, whatever) have major issues. The rubber bushings rot and the stamped cups that hold them in place rust out. The arm will rub on the frame bracket where it goes through, elongating the hole and wearing through the arm itself. Don't bother with factory replacement parts on this (except the arm itself if you really need it). For the same cost you can make it better. A bearing cup and washer welded in place and urethane bushings will fix it forever. I've done it on two D21s and three Pathfinders and every one of them handled better afterward and show no signs of having that problem again. There's a decent forum post about it with pics here:
http://www.infamousnissan.com/forum/showthread.php?t=37835
Tie-rod ends and adjusters are a bit of a weak link. They can be upgraded with aftermarket parts but, frankly, it's not worth it for the price. Keep a few stock spares on hand if you hit tough trails. The centerlink, however, will get sloppy and can hit the frame under stress. Keep an eye on it. An idler arm brace is also a very good insurance policy. I built mine but they're only about $30 from
http://www.4x4parts.com/. That's also a good source of other HB and Pathy parts.
If you crank the torsion bars get aftermarket upper control arms from 4x4parts.com. They're worth it to save upper ball joints and the price isn't bad. If you ever want more than 2.5" of lift you're better off going to a solid axle swap or get a Toyota. Nissans are every bit as tough and dependable as Toyota, but the 80s and 90s models don't have a lot of aftermarket support compared to Toys. Check forums such as roninwheelers.com, infamousnissan.com, and pirate4x4.com. A lot of Nissan guys have gotten pretty darn creative andcan do a lot with these trucks on the cheap.