How about this: With a second gear ratio of 1.68 to 1, that should mean that I have the same RPMs in second gear at 30 that I have at 50 in 3rd. If it vibrates at those RPMs in second then I've got an imbalance in the motor/flywheel/pressure plate. On the other hand if I run the truck up to 50/55 and then depress the clutch, vibtation must be coming from a bad u-joint/torque tube bushing/wheels or tires.

It may not be the easy way, but it is the Cowboy Way - Ranger Doug
Beware of the stories you read or tell; subtly, at night, beneath the waters of consciousness, they are altering your world. - Ben Okri
1953 Chevy 3100
1960 Volvo PV 544
1941 Chevrolet Special Deluxe