CMuc - my name's not Benzmac but I can answer you on this one. A bad strut on my 300E caused exactly the feeling you describe. I was able to swing the rear end left and right using the gas pedal - very scary. I identified the bad bushing by jacking the car up and pushing the wheel around with my hands while looking at all the links. Bought the link for $35 mail order and replaced it myself in 1/2 hour. Put the car back down before you completely tighten the link and then get an alignment as Benzmac says.
P.S. Obligatory plug, they sell parts here!
99 E300 TurboDiesel
86 190E 2.3