I had a 1973 220D with a whiney rear. I am pretty sure your car uses the same rear. I believe those particular rears were prone to this. I bought a used rear and installed it, and damned if that didn't whine worse. So I bought another used one and installed that and everything was fine thereafter.

Installing one is not rocket science, but it's a half-day job you don't like having to do twice. Back then (1985) new ones were 900 dollars. I should have bought one to start with. I would guess new ones are no longer available.

I wouldn't try to work on one of those third members. Replacing bearings and such. Getting them set up and the clearances adjusted is a professional's job. You really need to know what you are doing.

Richard