go underneath the rear end of your car and grab one of your real axle shafts and try and make it wiggle (side to side) because I had a rear axle that had good boots on it from visual inspection but once I grabbed and wiggled it I realized that it was going bad and fast. once I swapped out the axle no more vibration or weird clunks at low speeds.

you have told us the condition of your rear sway bar bushings and whatnot but what about the axle?

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'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.