Troubleshooting a Vibration ?'s

[dznucks]

I have a '69 C-10 with a mostly stock 350 and a 3 -speed column shift. I have developed a vibration that you can slightly feel at idle and gets worse around 1100 rpm, settles out until up around 3000 rpm and above. New plugs and wires and the engine runs good, so i doubt its a cylinder not firing.

the vidration appears to be in the motor or is directly related to motor RPM. it has nothing to do with being in gear and the truck doens't have to be moving.

I pulled the clutch dust cover and turned the motor over while inspecting the clutch assembly. nothing out of the ordinary. i watched the front of the motor while idling at 1100 and the pulleys and balancer appear ok. It kind of feels like its coming from the back of the motor.

the question is what has everyone else found as a root cause for this type of symptom? I really dont know what else to look at other than the internals of the bottom end.