Well to help add to the empowering of others in learning about their fine MB automobiles, I will post the conclusion of this 9 month vibration problem!
Turns out the mechanics loosened and then re-tightened all components that support the driveshaft, and this solved my vibration problem. The flex disk(s) and other support stuff was fine - they narrowed it down to something in the driveshaft, not the front end. Since everything was in good shape, the last ditch effort was to retighten everything, and that was it. The car drives like a dream now. I wish I had known this about $2000 bucks ago, but the problem puzzled several top-notch mechanics. Thanks again to everyone on here for your help. I certainly hope others can learn a bit from these posts. I sure did!
Ft. Lauderdale, FL