did they look at the engine mounts ? sometimes they go bad or get loose and the engine will vibrate the car at highway speeds.....
Not sure if they checked those. The problem is the vibration is intermittent. Doesn't happen all the time, and it's sometimes worse than other times. It's very frustrating.
It feels like it's originating from the rear. I believe the next step might be to replace the rear diff to see if it's the cause.
They replaced the drive shaft because they road tested the car with it taken out and it seemed to disappear. Logical thought process, but after a couple days, the vibration returned.
This is the weirdest thing I've ever encountered with a vehicle.