2500s 2wds have torsion bar suspension and the frame is the same as the 4x4 (except some early 99 and 2000 2wds have coil springs). The mounting points for the front diff aren't there but they can be added easily. Same thing with the Tahoes.
Adding a manual transfer case wouldn't be too hard, but still a pain.

That being said, if you are in the market for a truck, I would just buy a 4x4 to start with. By the time you factor in buying all the parts to do the switch and the time involved, it would be cheaper to just find a 4x4.