Start by jacking the car up so that you can remove the wheel. Secure the vehicle on blocks to be safe. Remove the wheel and examine for obvious problems. I would be inclined to start with the dashpot / shock absorber. Grab it and shake it to check for motion or noises. What you are checking for is failure of the bushing at either end of the shock or loose mounting bolts. 140s built prior to 1972 or 1973 were notorious for the rear shock mounting bolt coming loose in the trailing arm allowing the shock to bang around. There was a retrofit kit with a revised mounting bolt; but, you vehicle may not have received the upgrade (Volvo was not particularly proactive about applying fixes).
If it is not the shock that is the problem, examine the big bushing on the front of the trailing arm (if it is original it should be toast by now) and the bushings on the front and rear of the torque arm ( the rod mounted on top of the big trailing arm). Also, check the bushing on either end of the panhard rod (rod that diagonally connects one end of the rear axle to the body at the either end).
If it is not the shock or the bushings, things are more complicated and likely more expensive.
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