Vibration Problem

[70sixpkrt]

When taking the off ramp of a freeway at 55 mph and pushing the clutch pedal to the floor and foot off the gas pedal I can feel a vibration from under the floor board. Does that sound like u joints, driveshaft? I did replace my clutch last year and did not balance the pressure plate and flywheel together. It is a Mcleod Borg and Beck. I was told I did not have to since the pressure plate was neutral balance. I did resurface the flywheel.