[1 Lucky Texan]

could you listen back there outside the car while someone else backs it up? See if the sound is at a wheel or more central.

could be something with a brake pad/caliper or the parking brake parts inside the rotor.

also, there are bushings at the differential mount that could be destroyed. control arm bushings too.

You said the ujoint was OK, there's also a carrier bearing. dunno if it could clunk though.

at that mileage - I suppose a rear CV joint could be bad.