I have a 1987 560sel. My car car just recently begun to vibrate upon acceleration and when the car is stopped at the stop light in knocks. It also was experiencing high idle. I took it to a friend yesterday. He replaced the overload relay. The idle went normal and the vibrating ceased a little. As I drove it a little it started to vibrate more. Could it be something else that could be causing this?

Have the drive shaft center support checked
To DIY you need to lower the muffler by releasing all the muffler hangers
dont let the exhaust hang use a jack stand to support it in a lower than original spot then you'll need to remove the driveshaft heat shield and visually inspect the center support.It is in the middle of the drive shaft its round rubber bearing carrier that wears over time and sometimes just breaks!
Grab it and try to move it up and down. If it moves a bunch its bad and could be your vibs. HTH
Tech1

Thanks I will try it.