Ok I know this is a late chime in and I know you've already sold your truck but I just thought I'd throw in my $.02. First off I4's are actually good at turning high RPMs because there are only 4 pistons moving in one direction instead of 6 or 8 in almost opposite directions. I4's are EXTREMELY easy to balance, most are balanced well from factory already, so they will turn HIGH, HIGH RPMs. Look at Honda's V-TEC at stock in 1989!!!! the redline was 8 grand and you can easily take that up to 10 or even 12 with somebody that knows what they are doing. Although you would have to have a cam that could make power there.
Second, STAY AWAY FROM ROTARY!!!! Unless you plan on spending every day maintaining them then leave them alone. Yes they can last a while but if even one little bit of maintenance is missed then you have a blown engine on your hands.
If you want to go with an oddball engine go with tbe Toyota V8 found in the Lexus SC400. Yes I know it's a V8 but one that you won't really ever see in a LUV. They are only 4L and are easy to mod being Toyota. Also, (Don't quote me on this) I'm pretty sure Toyota has some 4x4 transmissions that will bolt up to it. The main downside to this is that it is EFI
But it would make a hell of a 2wd truck!
Another good swap would be the 4BD1-T. It's a good 3.9L turbo diesel that there are quite a few upgrades for plus (here's the best part) adapter plates are sold to bolt it to ANY Chevy transmission that will bolt to a 350 4.3 V6 etc. etc. I personally eventually plan on doing this swap into a 4x4 LUV hopefully relatively soon.
This was just my $.02 of advice consider it almost worthless