I know the 4wd brackets mount right in the middle of the spring hanger/core support bracket on the frame and brackets have a recess in them to clear the turn signals. From what I have seen the 71-72 2wd do not have the recess. Here are the 4wd brackets.
https://www.classicparts.com/1971-72...info/65-355B/# I put 2wd core support frame brackets on my truck because I do not need the spring hangers. They do not have the hole in them for the bumper bracket and have to drill out. Suspect outer 2wd brackets are longer and mounted further back on frame.