There are several ways to go about this, and all have pros and cons. The key to all of these is that the frame itself is different between 2wd and 4x4, specifically the engine cross member. A 2wd one slings much lower than a 4x4 and the mounting points for the axle beams are wildly different.
Probably the easiest is to do a solid axle. This requires a lot of custom fab work, and axles with the correct width and bolt pattern that are strong enough to support the front of that truck are hard to find. It also usually requires at least a few inches of lift.
If you want to do with what would have been a factory TTB axle there are two other options.
You can get a 4x4 engine cross member and put it in your frame, or you can just get a whole frame and move your power train, body, and any running gear you want to keep over to that frame.
As counter-intuitive as it may sound the second option is much less labor intensive, and you could get the new frame all painted and sealed up before moving anything on to it while still driving your truck, cutting total down time drastically.
Now lets look at what has to be done to get one of these engine cross members out.
They are held to the frame by 8 stamped rivets, four top and four bottom on each side. They are also welded in four spots, one front and one rear on each side. The driver's side of it is also under the steering gear reinforcement bracket, which is held to the frame by about 4 feet or so of weld, about 2 feet top and bottom each. All those rivets and welds have to be undone. I have done it with the cab on and off, it is much easier with the cab off. It is not possible with the engine in, as the engine sits directly on the piece you are trying to remove. You also have to have the front axle out since the beams mount to this same piece, but you will have to take the axle out eventually anyway.
On the other hand when we put the truck that I used to learn all this back together, I had the engine and trans sitting back in the frame, all my wiring laid on the engine, got 5 of my strongest friends together, and we just picked the cab up and set it on the frame. Doing it that way was much easier with the bed off so you can walk up from behind and don't have to try picking the rear of the cab (the heavy end) up far enough to clear the engine you just tip the front end up a bit. So moving a cab and bed to a new frame is not all that hard.
I have done this, and if I was starting that project again today, knowing everything I know now (namely that I never want to drive the truck when there is salt on the road after all the work I put into it) I probably would have just left the final product be 2wd, because now I never want to drive it in 4x4 appropriate weather.
Hope all that helps.
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