Like the title says, I'm getting a thumping type vibration frm the rear of the car, but only when it is under load. Car is a 1983 300D with 336K miles. If I accelerate gently from a stop or am cruising, no problem. But if I accelerate briskly or am going up a steep hill and give it some pedal, I get the thumping vibration. Once it starts, if I let up on the power it goes away, give it more power its back. Once the load is gone, all is ok. Doesn't feel like tranny slip as the RPMs never fluctuate. Rear shocks are a year old and look fine, rear sway bar and end links are fine, and driveshaft carrier bearing is new (car was starting to thump before it was replaced anyway). Transmission mounts, rear diff mount, and motor mounts were all replaced to eliminate the problem, but no dice. So, could it be the rear axles? No visible tears or leaks in the boots, but they look original to the car. What about the U-joints? Help!