91 explorer

when pressure applied to brake pedal i get an awful clunk and then when brake pedal released it happens again.anybody ever heard of this before?it's drivin me nuts.i'm thinking it has something to do with the rear wheel anti-lock brake system(rabs).HELP

This is my opinion, but sounds more like a mechanical problem other than RABS system. Sounds like you have a broken pin holding the rear shoes, or something else similar. Get it looked at ASAP or pull the rear wheels and look. If it is a RABS problem, not many people will deal with that but dealer, so look for broken pin, shoe, etc FIRST.