It is probably just your right rear sway bar link. It will clunk when it breaks loose from the bar. The more weight the more noise it make because it is closer to the body/frame.

Please check these first. Easy fix. I had to replace both on my 87 SDL.
Good luck!
below is the link to the part you would need.
1987 Mercedes-Benz 300SDL Base Sedan - Suspension, Shocks & Springs - Page 3

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Western NC
1987 300SDL W126 Turbo Diesel 335,000 + miles
1976 300D W115 275,000 miles+