Loud noise when stopping then the wheels locked up completely

After driving for an hour I pulled up to a stop sign and a very very loud noise was coming from the passenger side rear tire-brakes. I pulled into a Ford dealership and just asked them if they thought I could make it a little over three miles if I stayed under 30 mph. They had me pull up and then go on reverse. They just listened and didn't do anything else. They said yes. I backed out and went a total of three blocks and it just locked up completely.