This may help somebody. On my C55 Estate, I started to get a bad vibration at 55mph. You could not drive through it!! Off to the local wheel balancing people. All wheels checked out nicely. Still the same issue. Problem was getting worse when under load (vibration was more like a shake) and going up hill was not pleasant at all for my teeth fillings. Prop Shaft Flex Disc (Donut) had started to break up. New disc from ECP - which had to come from FEBI. Fitted by Wright Tec yesterday. Job done no more vibration. When the old disc came out it pretty much fell apart. So if you have a vibration coming from under your seat that gets worse under load - check out the flex disc.