There are some threads covering this subject already, but here's a quick rundown of some of the things this involves:
The 2wd tranny crossmember is welded to the frame. A 4wd cross member will need to be bolted on to some fab'd mounting plates. Because of this, I think sourcing a 4wd tranny/t-case will make it easier to get it positioned / installed.
Get driveshafts off a similar body style 4wd truck, or have them made.
Swap rear axle from a 4wd, to match bolt patterns, also to get the 8" 3rd member.
You'll need to use a 4wd IFS power steering box for crossover steering. IIRC, the 2wd version doesn't work for that.
Hope that helps.
Todd