1987 Mercedes Benz 190d car sways on highway

I't my fisrt time on this site! My 190D 2.5 has started to "sway" when driving on the highway.

I've been bringing it to the garage and after more then 2000$ of replacement on the supensions parts, it still has that "spongy" feeling in the back when driving. The car seems to be rolling on rubber on the rear of the car. It's very unpleasant to drive on long runs...

Does someone had the same issue and what has been done to repair? I love the car and would like to get back the driving experience this car was offering!
Thanks!