1989 BMW 325 bumping sounds coming from the rear axle

Sorry to say, but sounds are notoriously difficult to diagnose from cyberspace, could you be more descriptive?

So, I'm cruising down the road, at say 70mph, and let off the gas - it clunks as the torque is forced to dissipate thru the system. Anyway, I'm concerned if this is an axle, or the rear end, or something like that. I've had the car 6 mos, and it always goes, with no trouble.. Obviously with this sound, it might die soon, or finally actually break. Is it possible that I just need a new rubber block above the rear end? Similarly could it be a u-joint that connects the drive shaft to the front of the rear-end.. What do you think?