Sounds like a bad Drive Shaft Flex Joint http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=26R0X2XXH26R0X3FUV&year=1988&make=MB&model=300-SEL-006&category=K&part=Drive+Shaft+Flex+Joint
Or a bad drive shaft. A drive shaft is the longest bar of the car that connects the transmission to the rear differential.
BTW your car has two Drive Shaft Flex Joint, one front and one rear.

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380SE 1984 <------- Totaled by flipping accident
300SE 1988 <------- Sold
Volvo S70 2000 <---- 4 family
Opel Omega 2002 <-- 4 family
560SEC 1989 <------ Sold
560SEL 1990 <------ Sold
560SEL 1991 <------ 4 my mother
560SEL 1989 <------ Sold
600SEL 1992 <------ Current

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