Here is the in-depth write-up that I hope can serve as a how-to for OM617-swapping 96-04 Tacomas/4Runners (and T100's).
I picked up this $600 1997 4Runner 5VZ/A340 4x4 (rear e-locker) with roughly 250k miles and it was SHOT! Completely rusted out even though the body doesn't show it. Destroyed interior. Radiator blew up and mixed trans fluid with coolant to make a milkshake in the trans. Now I haven't read a single report of anyone flushing their trans and successfully driving it after that. Even the ones done under warranty at the dealership with the power flush had trans failure shortly after. Keep an eye on your radiator if you've got an automatic! The PO claims he noticed it right away and had it towed home where I picked it up. I flush the trans, added a new radiator, and put another 30k on the 5VZ! I daily drove it to work, used it to tow the mower trailer around, and even drove it 3 hours into Iowa with the trailer to get a band saw. Did a fair amount of wheeling with it, too, enjoying the rear locker. Eventually, it started losing coolant, so the 5VZ got yanked and sold.
The plan for this has always been for R&D. The only money I have into this rig is the new radiator and mounting the free tires someone was throwing away. Sold the 5VZ to cover the purchase. Can't beat that! I'm currently swapping in an OM617. I'll be using the 4Reigner to test trails I'm cutting in our pasture, as well as haul the tree cutting equipment around. Since the frame is already falling apart, I don't expect to keep this around much longer. I had originally planned to do a TDI swap after the 617, but have since decided not to worry about making TDI adapters for the Toyota Automatics, as the torque converter pilot stub would require turning down, and that is probably too much effort for most people. Once the frame folds in half, I'll pull the 617 back out to go into something long-term, and I'll save the rear axle and trans.
Brake upgrade time! Any time I'd hit the brakes above 15mph, the steering wheel and entire suspension would shake violently. I got in the habit of manually down-shifting to mitigate this, which is good now that I have to for the diesel driving, anyway. But I needed brakes... Did some research and found a 2001 Tundra brake package fits. The smaller brakes can fit the 4Runner 16" wheels, but the larger 231mm package requires 17mm wheels. The brake price difference was negligible (they were both expensive), so I went with the bigger ones, even though I hated the idea of buying yet more wheels/tires since I have tons of 15's kicking around.
I got Powerstop's 2nd best package for $350 with a bunch of discounts and promos. Found some primo 5th gen wheels for $200. Got some NOS tires off craiglist for $420. Had them mounted/balanced, got them back, threw the first 1 out of the truck and weights came off, so back they went to the tire shop. The idiot didn't even wipe the wheel at all - just tried taping the weight on over top of brake dust and dirt. They're good now, though. I'll be burning off my excessive supply of tires in the rear, hence why you'll see my pictures with weird wheels in the back.
The swap was completely painless. Somehow, all the hardware came right out, even the brake fittings, and bleeding went smoothly. I went to bed the brakes in, getting that nice blue color on the rotors, and one of the rotors began showing a casting flaw. This led to some major down-time (2 weeks). Eventually got the bad rotor replaced and with 600 miles on them now, they are freaking awesome!! Hands down, should've done this the first day I bought this 4Runner. I can damn near do a stoppie.
I also installed some All Pro Off Road rock sliders. They were selling them for the same price it'd cost me in materials, so it was a no-brainer. I also got tire of hearing people poke at the damaged front fender and gorilla tape, so I used my new stud welder to yank it out.