Vibration, I think I've tried it all, no luck

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I have a 96 Formula convertible M6. It has a VERY strong vibration at 145 KPH (about 90 MPH) The car has an aluminum driveshaft (changed years ago under warranty) I had the wheels balanced, no change. Then I checked the trans mount, it's good. Then I installed a Spohn torque arm and confirmed that the pinion angle is within 1 degree of the trans angle. This actually made the vibration feel worse, but I think it's because of the new mounting location and poly bushings. Next I changed the U-joints and had the driveshaft balance checked. Still no change. Finally I switched rims and tires to some old camaro take-offs that I had with Bridgestones on them. Still no change. What else can I do? The car drives well other than this, and the vibration dissapates a bit over 160 KPH (about 100 MPH) but at 145(90) it's VERY pronounced. Any ideas?

[Crimson Sin]

Hmm I don't know if it will help, but I had a similar issue with my old 93 Cobra, made me crazy for about 9 mos, then one day a really old grizzled tech laughed at me and said change yer tranny.. I laughed and went on my way, 2 mos later and a new DNE ( Doug Nash Engineering) trans, vibration went away, speed shop I took it to said the undermain shaft was bad or somesuch, this is about10 years ago so the details are a bit hazy. Just something to think about..