do you use your emergency brake?
if you do, park the car without the emergency brake and test again.

WD40 is more of a cleaner then a lubricator

you can use the other forum here to help find a mercedes shop familiar with your car.

you can also use FASTLANE here to familiarze yourself with proper shocks.

how many miles are on these shocks? and research having most of the rubber rear suspension bushings changed out