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.
I wish it was as easy as that. When you're dealing with vibrations, you have many different forces coming from several components. Some vibrations require harmonics from more than one source before you hear or feel them. Adding to that are road conditions. I had a bad wheel bearing which only caused trouble when I was driving on a local road which happened to have a high crown in the center, causing a lean toward the curb.
My approach for trouble shooting vibration issues is the same one I use when shooting pool. Shoot the easy ones first. Checking wheel balance, bulging tires, loose wheel bearings, too tight front engine motor mount, loose u-joints, are easier to check than engine, clutch, transmission, and rear end problems.
Carl
