will an 86 f150 2wd swb body fit on an 81 f150 4wd swb frame?

 

If we're talking about switching a cab or a bed, yeah sure. You'll have to cut a hole for the transfer case lever, but I'd bet it's factory marked. 80 to 86 body parts are the same 2 or 4 wheel drive except maybe some different bumper/grille/trim options.