Basically what you need to do is watch Trucks and watch Stacy David build Sgt Rock. He more or less builds a 4wd monster truck out of an old army 4x4 Dodge. You'd have to put a straight axle in the front end and build brackets to tie it to the frame. An Atlas transfer case can be run external from the 4L80E the same manner that monster trucks do. You'll have a chunk of change spent building it but shopping for all the cool parts is half of the fun. You could buy a 4wd drivetrain from a salvage yard and swap frames. The NP205 is a gear drive unit that GM hasn't put in trucks for 20+ years. That's one tough unit. I had one in my 73 and it ran dry without lube for 2 years before it finally self destructed.