Harmonic balancer issue?

[Larsen74Dart]

'74 Dart Swinger, 318, with a very subtle but consistent vibration in car. Had tires balanced/rotated, helped a LITTLE, if at all. Friend suggested harmonic balancer and upon visual inspection, it appears the rubber component is dry and cracked. Can't get a pic of it right now. Could that be the issue? Anyone have any experience with this? What is a good replacement part?

 

[RustyRatRod]

Possible. When is the vibration present?

 

[cudamark]

Does it vibrate just sitting still in Park/Neutral and rev'd up? Only while driving down the road? What speed? What RPM?

 

[Larsen74Dart]

Does it vibrate just sitting still in Park/Neutral and rev'd up? Only while driving down the road? What speed? What RPM?

All the time. Idle, rev'd, all speeds. Stops when I turn it off...