I agree with Bearcuda, see if there is a tire shop with a road force balancer near you and start there after checking the common stuff others have mentioned (pinion angle, engine and trans mounts, loose exhaust hangers, u-joints etc) Also, if you take the car out of gear at that speed, does the vibration change in any way? See if there is a tire dealer that carries road race tires since they run sustained speeds, they would have some different experience I would think. Could be some weird harmonic convergence thing maybe, but that is getting way out there....
