I had a similar problem with a new MB E500 that I purchased in 2003. The car would vibrate at speeds between 60-80mph. I took the car back and had to go through the usual first line of BS "our tech has looked at it and there is nothing wrong" I knew there was a problem as my Bro had the exact same car and he did not have the vibration. This discussion goes back and forth a couple of times and I demand to meet the senior tech at the dealership. I go for a ride with the techie and he tries to convince me that the ride is normal, but he is only doing 55-60. I insist he takes the car on the 405 freeway and drive at 80mph. At the speed the vibration was so obvious that he turned red face and admitted there is a problem. So the car goes back into the workshop for two weeks and I manage to build a rapport with the tech guy and I tell him that I have complete faith in him, however if the problem can not be fixed would he support me with discussions with the dealership about a replacement or unwinding the deal? Amazingly he replies yes. Two weeks later the problem persists so I arrange a meeting with the GM and the tech guy. He informs the GM the car cannot be fixed he has tried everything, the GM tries to give me replacement but I decide i want nothing more to do with MB and demand they unwind the deal and not obligate me to take a replacement of their choosing. Luckily the GM agrees
After the saga I went for a FX35, absolutely amazing car got a second one 3 yrs later. After the second one I wanted to try something different I loved the shape of the new 750 LI series and decided to test drive it, fell in love with the power and smooth ride. I absolutely love my car, there are not many where I live and so I get a lot of looks when driving. I will definitely get a second one.