Question about putting 4x4 into a 2x4 vehicle

So I'm just wondering if anyone has done this? Putting 4x4/AWD into a vehicle that is either a RWD or FWD vehicle, it seems like a big job, and can it be done? Is it more cost effective to just buy new, but in the case of a project car, how could one go about it? via /r/cars http://ift.tt/2u67ZCX