Last time my '85 300TD did this, it was a broken rear sway bar link. They're plastic, cheap, and easy to replace. But you've got to get on the ground or pull the wheels to check them.

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Dale
'67 Volvo 122
'79 Rabbit Diesel
'79 M-B 300TD
'82 M-B 300D
'83 M-B 300CD
'85 M-B 300TD
'86 Isuzu P'up TD
and a couple of Hondas