vibration -Harmonic Balancer?

93 Chevy Silverado 4x4 has a vibration I can feel in the floor of the truck at cold idle and while driving at 5 to 55MPH, at 60 to 65 MPH I can feel it in the steering wheel and seat. Garage thinks it’s the harmonic balancer rubber piece associated with the balancer. Is that correct? I don’t understand all the components of the Harmonic Balancer.

Could this be true? And how can I detect or find that it is this? They want to replace this rubber piece and see if that will fix the problem.

So, you can feel it when you're driving, but can you feel it at the same rpm's in park sitting still? From idle slowly increase rpm's up to about 2800. If you can't you've just eliminated an engine problem. The harmonic balancer bolts to the front of the crankshaft. It is typically a 3 piece unit. The inner metal piece is what bolts to the crank. Then a rubber piece and finally the outer metal ring that has the ignition timing marks. Sometimes the rubber between the inner and outer metals goes "south" and the balancer cannot "absorb" the harmonic imbalances of the motor. I've never heard of the rubber being replaced, just the balancer itself. Hope this helps..........Steve