No if you had a bent shaft (unlikely but possible if car was rescued and this applies to just out of balance shaft as well) the crucial speed is not the engine or gearing speed prior to the driveshaft. The only time the driveshaft turns at the critical speed is when the rear wheels are turning 70mph or above if it is the driveshaft problem. The ratio of rear end to driveshaft is fixed. But does make me think of something. (usually takes dynamite) Try driving car at vibration speed on downhill grade. Shift automatic to neutral and make sure vibration still persists or not as may give a valuable clue. Also when you get car on hoist to inspect things just do not look. Pry things and push and pull on areas like your centre bearing. Make sure bearing on yoke end of transmission is not abnormally worn. Any wear that causes the driveshaft to run offcentre is going to induce imbalanceas well. You do have this clunking noise that may or may not be related to your vibration problem. As you are driving a car with a lot of miles on the chassis you have to keep an open mind. Even a shot transmission mount might cause some symptoms like you describe. Another thought is to let a mechanic with a lot of mercedes experience have a quick look. Things that vibrate excessively cause wear. May be cheaper to localise problem earlier than later. Would be an easier world if everything was straightforward. Experience teaches us to expect even things that seem to almost defy logic from time to time.