Just to eliminate one thing, take off your rear wheels and the drums and check to see if there is oil on the pads from a leaking axle seal.
One of my rear seals were bad in my truck and it contaminated the brake shoes and caused them to lock up on me at slower speeds. There was no difference at higher speeds only when coming to a stop at under 5 mph would it lock up. Don't know if this is the same you are experiencing or not but just thought i would throw it out there.
Oh and the fix, I replaced both axle seals (to be thorough) and both of the rear brake shoes, and drained and refilled the rear differential. Fixed the problem out right.
assuming you have an automatic, if you put it in neutral when braking hard and do not experience the clunk, then it almost definately is the slip yoke. supposedly it's "normal" though i wonder what effects it has long-term.