Will a 91 GMC Burb 2x4 body fit on a 86 chevy 4x4 3/4 ton frame?

You may have a problem with the transmission tunnel. The 4x4's have a taller tunnel (high hump)than the 2wds (low hump). You could probably fix that situation by using a 2" body lift.

yes the 91 body will fit. i have a 88 2wd body on a 83 4x4 frame. we cut the 2wd tunnel out and and installed the 4x4 removable tunnel. makes it alot easier to access the transfer case up top. but a body lift should also take care of the problem.