If you have a manual transmission, can you take it to some hill, push in the clutch and turn off the motor? Don't do it on a crowded highway since your power steering and brakes will lose effectiveness.
If the vibration goes away it's in the engine. If not, it's elsewhere.

How about connecting the rear drive shaft and removing the front hub plates? That should allow you to tell if it's the front drive train out to the bearings at least.

Also, it could be a tire too. I had a tire on a car once that must have had some internal cordage defect which caused quite a vibration. I don't think the balance was affected but the ride was. Perhaps swapping your spare for each of the four tires.

Does the vibration peak at a certain speed or just get worse as speed goes up? Is it related to engine RPM or just vehicle speed?

Chip