Thanks for the tips.
I just returned from the local MBZ shop, where they test drove the car and then raised the it on a lift and inspected the drive train. The mechanic found the flex disks to be in replaced/good condition but the dif and rear suspension bushings very worn and in need of replacement. It was explained to me that the worn bushings allow the rear drive train to flex under torque, thereby throwing the drive train out of alignment ..... causing the 'seat of the pants' vibration. I hope this relatively simple repair is the solution to the vibration problem.