I'm pretty sure the frames aren't different other than maybe the front crossmember under the engine? That's a simple fix though just shear off the rivets and install a 4x4 crossmember I would just go aftermarket myself.
The biggest task of course is mounting the leaf springs and hangers to the frame, im not sure if the 2wd frame will have the mounting locations (like bolt holes, ect) if not you have a lot of drilling to do lol. You have to make sure you get them mounted square and both springs are mounted evenly with each other so you don't have a steering/tire wear issue, if you want to call it dog tracking (what its called with rear axles).
You'll need a crossmember for the transfer case, and you'll have to cut a whole in the floor for the 4wd shift lever, driveshafts, and you HAVE to have the same gear ratios in both axles. You may have a little headache with the yoke's on the driveshafts and axles being different also.
Its been done many many times and ive considered it myself, I want a crew cab 4x4 but all I can ever find are 2wd and the 4wd's are wayyy overpriced when I actually find one. It can be done there's just a good bit of work that goes into it and the little things you have to do add up
