I'm running a Yanmar turbocharged 4Cyl Marine Diesel built to run Bio Diesel or fuel mix's, backed by a GM 2 spd powerglide trans, twin Dana 300C clocked and mated cases, a Ranger overdrive and Unimog 419 axles on my 90 Toyota. Over the last 12 years this trucks had many different drivetrains but ones the best so far.
The problem installing an engine like this is the adaptors to mate everything up are a little expensive, I've got just over $5200 in adaptors to make it all connect together and that was at discount prices through friends, luckily I was able to salvage the engine for free from a wrecked work barge and the axles very cheap from a burned Unimog otherwise this combo never wouldve come together.
Look at Advanced Adaptors and do an internet search for adaptor kits to install Diesels, this might help you with making a decision on which engine to install if you decide to go with a diesel, I highly recommend a Diesel engine for an off-road play truck. If it's gonna be your daily transportation I'd recommend looking at either building up your present 22r (LCE and Wildyoats both have a great variety of parts to increase the output of your present engine) or look into a Chevy 4.3 with a 700R4 trans, the marine 4.3 is a super simple swap if you get a carburated version because of their simplicity.
Also you'll need to figure out your gear ratio, I feel thats probably one of the biggest problems with your performance at this time. Keep in mind when that truck came from the dealer in 1987 it had 28" or 29" tires and only pushed 97hp, 31" tires dont sound like a lot but your engine is probably only making horsepower in the mid 80's range at best after all those years of use and abuse.
An easy way to guestimate your gear ratio is lift the rear tires off the ground, make sure the trans is in nuetral, make a chalk mark on the tire and a chalk mark on the driveshaft, rotate the tire 1 full revolution while counting how many times the driveshaft rotates, I'm guessing the driveshaft will go 3 3/4 of the way around, putting your ratio at 3.73 or something close to that, 4 1/4 around would be something like a 4.11 or 4.10.
Your desired gear ratio for your truck is gonna depend on how you want to use the truck, if you dont mind having to drive a little slower on the highway then numerically higher gears (like a 4.56) might work for you, but if you want speed then a 4.11 or 3.73 might be the best choice. Also keep in mind when choosing gears where are you going with this truck build, are larger tires in the future? That'll make a difference on what gears to choose.
