I had a similar problem, a vibration at 50 mph in 3rd or 4th gear under acceleration.
The dealer replaced the drive shafts under warranty but only after 2 previous visits when I was told the tires were worn out ( they were but I still got an extra 5000 miles out of them) Then they tried some new wheels because they thought they were bent.
Previous to this I had vibration under braking and the front rotors were replaced again under warranty
