These have both been mention here, but here is an easy way to check three things that can cause this.

Take your car to a tire shop and ask them to look at your tires. A good tire man can just look at a tire and see a slipped belt. If he says your tire is seperating then just buy a new tire and be glad that is all it is. The weird thing about tires is that you can have a bad one on the back and it will feel like the vibration is coming from the front.

Check your carrier bearing. Your driveshaft is in two parts (some Mercedes have three parts) and where they are connected under the middle of the car is supported by a carrier bearing. If you can move the driveshaft up and down in the carrier bearing ANY then it needs to be replaced. It will also make a Thump, Thump noise when you accelerate if it is bad.

The carrier bearing is really two parts: The bearing itself and the "carrier" which is also called the housing or the spider. When one wears out you just replace them both. This is not hard but it is time consuming.

A word of warning here. Mark the positions of the driveshaft (the front part and the rear part) to each other so it can be replaced in the same orentiation that it was in before it was removed. These are balanced as a unit and should not get out of sync with each other.

Also take care to not knock off any of the little square balance weights that are up and down the shaft. If you do you can put them back on with super-glue.

If you can push up on the rear of the transmission ANY the rear motor mount needs to be replaced.

If you have to replace your carrier bearing then replace the rear motor mount as well. It is not that big of a job once you have the driveshaft out.

Pooka