Block the front tires back and front.
Jack op the rear axel and put it on 2 good jack stands with the tires about 2" off the ground. Don't jack too high because it will change the balance of the car.
Start it up and put it in high gear.
Slowly run the speed up into the range where the vibration occurs.
If you can recreate the vibration, you eliminated the front end.
If nothing you can do will cause the vibration, start looking at the front end again. Swap the tires front to rear and do the test again. Still nothing?
Look for strange things to do with front end geometry. Are you running radials? does the recommended caster increase for radials.
bad or loose shock on one side? All the A frame bushings in correctly and tight?
