I know that GM had a couple of different bell housing patterns out back in the day. There was the Chevy bolt pattern and what they called the BOP pattern (Buick, Oldsmobile, Pontiac). As long as the engine bolts up to your transmission, you should be able to make it work.
There is essentially no difference between 2wd and 4x4 engines that I am aware of.
The problems I see with installing something different than what you already have in your truck is the engine control module (ECM). Installing any fuel injected engine with a different displacement or fuel delivery system will require swapping ECM's (computers) and a lot of wiring work. It's not impossible, but it would be a lot of tedious, brain numbing work.
<font color=blue>Daddy always said that a Phord is about as useful as a shovel with a rope handle.</font color=blue> [img]/wwwthreads_images/icons/wink.gif[/img]