changed rear flex disc-what a difference
 

I changed my rear flex disc on my 90 300d today. I shouldn't have waited so long but I got lucky I guess. One nut was loose and bolt was tilted a bit making the rubber bend. That was the vibration around 25 mph, I suppose. Also some pretty major cracks. I had looked at it about a year ago from the front side and it didn't look too bad. Anyhow, it's worth looking at from both sides with a bright light and pushing on it to check for cracks. I had two difficult nuts to remove, but struggled and won out, but two of the bolts were seized to the flex disc, so I used an oscillating multi tool to cut them. This was amazingly fast, like less than 1 min because you can spin the driveshaft for the optimal angle. It drives really smoothly with the new one. Hope this helps those who may be putting this job off on their car. I got the Febi Bilstein one that says "SGK" (Which I have read means that it is high quality OEM).

Good deal.

Just about the only part Febi makes that I would recommend is the flex disc. As you correctly state, it's a rebadged SKF disc which is an OE supplier.