Bad Harmonic Balancer?

A bad balancer is usually worse than that (it can be really violent) and when it goes, most engines never recover. Many times the engine only lasts a few months more.
When in doubt, replace it, a ford racing replacement doesn't cost much.

As for the vibration, if the vibration is still there with the clutch depressed while the car is parked, the problem is the flywheel forward. So that rules out the driveshaft.