i've done it the other way around, i.e. put all 2wd gear into a 4x4 shell.

3drstretch has it spot on. i would personally also change the complete wiring loom to make sure that it is all compatible, otherwise you may have some troubles (particularly the abs that uses a different ecu).

edit: there is one section of floorpan that is different on the 4x4 and 2wd. the piece has a mounting point for the gearbox on 2wd, but has a space for the transfer box on a 4x4. i would cut that section out of the 4x4 donor if i were you, and weld it in to the 3 door.