1978 Dodge Truck vibration

Hi,

I have a vibration in my engine. Well, at least I think that is where it's coming from. Anyway, I notice it at around 55 mph or so. It's not a real harsh vibration, it is more like a low smooth vibration if that makes any sense. I have been told that it could be my harmonic balancer. Anyway, it is concerning me that it may be doing some damage somewhere that needs to be taken care of before it I get too serious.
Any help would be appreciated!
Oh yeah, the mileage is on a rebuild.

Thanks and keep motoring!
Tankman