it is not showing any misfires, I have wondered if there was something up with the harmonic balancer or the friction plate at the rear of the engine
Harmonic balancer is a possibility I highly doubt the flexplate would cause this. When its vibrating you should be able to use a pry bar to put pressure on the transmission mount to see if that stops the vibration or even use your hand to see if you feel alot of vibration where that mount is if so replace. If not the next step would be to replace the harmonic balancer. You could also remove the drive belt and make sure it still vibrates to eliminate anything else as a possible cause. Let me know if you have questions, thanks.
Have you seen this in the trailblazer/envoy/bravada/rainier very often? The balancer appears intact and the rubber/plastic sleeve seems ok, is there anything I should be looking for on the balancer or the transmission cross mount?
One last question I promise. The vibration appears to only be noticeable at low idle. when it revs up it seems to go away, does this point towards the trans mount?