I've got a squeak aso on the '87 300D, so I got under the car for a visual inspection. The suspension arm (also called control arm) bushing on one side looks pretty bad. The suspension arm is a big arm of metal from the center of the car to the wheel hubs, that swings as the wheels move up and down. Near the differential (a little behind and lower) are the bolts that hold the bushings.
Because the one looks so bad I had not considered anything else under there - but there certainly are a lot of suspension components. So I can see how other things like the thrust arms could be squeaking too.
You could probably locate it fairly quickly if you get under the back and have someone push down on the car. I'd use the lip of the open trunk to push down on.
It is probably NOT the shocks!