Doing your own conversion isn't easy, but it certainly can be done. A few years back, I converted a 2WD Ford F150 to 4WD using all stock Ford components. I won't bore you with all the details, but here are a few pics to show what I started with, some of what was done and the final result.
This is what I started with.
Tore it down to the bare frame and cut out any 2WD components, including spring cups, and that huge 2WD crossmember that sits under the engine. That huge crossmember is where the IFS "axles" bolt to.
Some of the 2WD components that were removed.
Putting in the 4WD crossmember.
Bolting on the 4WD spring towers and bolting up the axle. 6" lift springs.
Frame horn from a 4WD frame.
4WD frame horn in place and ready to be welded.
Completed 4WD conversion, except for the steering box.
Steering box in along with a 460, C6 tranny, and NP205 transfer case.
Cab back on and hooking up electrical and plumbing.
The completed truck. I'm about 5' 10", so you can see how big it turned out.
This black truck was actually the second conversion I've done. The first was several years back with my '70 Ford Crew Cab.
The flat bed I built for it.
Converting something to 4WD can be a pain, but the end result is certainly worth the effort because you have something unique! With the black truck, I had something no one else had. Ford never made an extended cab short bed 4WD in this body style. All the 4WDs were longbeds, and all the shortbeds were 2WD. I had Ford truck enthusiasts everywhere asking where I got this truck. I just told them that you can't buy a truck like this - it has to be built. Even had a couple of guys ask me to do a conversion for them.