1996 Cobra has slight vibration at 48-52, and 64-70 miles per hour.  I have changed the front brake rotars, no improvement.  Then changed 6000 mile  BFGoodrich Kwds (showed no signs of uneven wear) tires to Michelins.  About 80% of the vibration is gone, but still experience some.  Vibration is there if clutch engaged or disengaged, so can eliminate everything in front of the clutch. Vibration is there all the time so can eliminate flat tire spots.  Changing tire pressure, or rotating tires show no change.....where do I look next?  Back rotars, Ujoints, drive shaft.  I bot this car new it has never been wrecked, has 75000 miles, and do not remember the vibration until aout 65000......  

Jeff
Recurring vibration like that is usually out of balance harmonics. Could be in the driveshaft the rear end or in one of the wheel/tire/rotor assemblies. You can usually tell if its in a wheel because it will feel like that, so my guess is the driveshaft is a good place to start. I personally had one like that which was cured when I replaced the pinion bearing and outer bearings on the rear end. Driveshafts can be tricky and I have also experienced vibration there from a universal that wasnt seated correctly, a burr or anything in the pocket will affect it. Have them check the u-joints when they balance it.  

I found the source with the help of mechanicalguy.  Took the drive shaft out, boy was it tough loosening the back bolts, Ford put 'em on for the racers.  Anyway when we got it out the back UJoint was nice and loose in all directions.  The front one was stiff in all directions, figuring that with Ujoints loose is better replaced the front one and the problem seems solved.  Drive shaft other that a little rusty wass in excellent shap, all the weights in place, not dents, dings, etc.  Thank you so much for the advice.