When troubleshooting a vibration, you must first determine if it follows the speed of the engine, the speed of the car, or maybe the speed of the driveshaft.
If it is the engine, you will probably have it when you reve the engine in neutral.
If it is driveshaft related it will be very high frequency, about three times as fast as if it were a wheel. Either one of these will be proportional with vehicle speed regardless of what gear you are in.
BTW, a ridiculously high percentage of vibration complaints turn out to be wheel/tire related. Just because they've been balanced does not mean they are not the culprit. First of all many tire stores won't balance Benz wheels correctly because they insist on static balancing with weights only in the inboard lip for cosmetic reasons. You will rarely eliminate vibration this way.
Secondly, tires can have stiff spots. The only way to determine this is by having them run on a Hunter GSP9700 machine. You can find the location of one in your area by going to: www.gsp9700.com.