Does anyone know of a source for rear bumpers for these trucks?

 

For a factory rear bumper, good luck, they are fairly rare. The brackets were different than the front, with a drop to make the bumper lower. The blade almost looks the same, it's the brackets that are hard, if not near impossible to find. If you are just looking for a good looking bumper, you could maybe modify original front brackets, or fabricate some to fit. The frames of these truck are SAE standard, 34" I believe, so it's not hard to get somethign to fit.