This is a long shot, but you guys should check your parking brakes. I had a rear spring break on me, resulting in the retainers for the brake shoes being partially ripped out of the backing plate. It would cause occasional binding of the shoes on the rotor which manifested as a pretty violent clunk. It was also sporadic and hard to reproduce. I replaced the RTABS to no avail and was losing my mind poring over all the rear end components trying to find the problem. Checking the parking brakes was a desperate last-gasp effort (seemed the least likely culprit) but ended up being the problem.
It's worth a look.