1999 S500- vibration

[MBD]

At around 30k miles I developed a vibration at speeds of 55 and above. You could feel it in the steering seat etc., visor would shake. I took it to the MB dealership and they recommended new tires. I replaced them with Michelin 235/60/16. The vibration was still there. I returned to the dealer they suggested that I had bought a bad tire. I spoke with the Michelin rep and he had someone look at the tires and check it out. They felt that it was the drive shaft (possibly unbalanced). I returned to the MB dealer and reported their findings. After doing their own checkup they agreed that the Michelin Co. must be right and replaced the drive shaft under warrantee. Still the same problem. I was then told that the tires must be out of balance...same problem. After several k miles and several $100 in balance and rebalancing I decide to ride it out until I need new tires. At 55k miles I was sitting at a light when I heard a series of loud pops and my tranmission dropped out of gear. MB dealership required me to buy another since it was out of warrantee and I still have the problem. I am on the road constantly and do very little in town driving. Do you have any suggestions? I love my auto and would like to feel good about the ride again.