A vibration at speed could be as was earlier suggested, the carrier bearing (center bearing), or something else in the differential. If the flex disk was bad enough to be vibrating like that, it would be about to let loose. Generally the front wears out before the rear.
To check the center bearing, you will need to open the bottom of the tunnel. Once the bottom plate is out of the way, you'll have a good view of the bearing and should easily be able to tell if the rubber has deteriorated.
I just looked at your car, almost 10 years and 72K miles, you might want to disconnect the front yoke at the flex disk, just to give the face of the disk a good look at. Check for any cracks in the rubber. If you see any, replace it - the disk is inexpensive but if it lets go at speed you will be in for a big surprise
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.