OK, so I already did a huge search and turned up nothing. My problem seems to be different than what everyone else is experiencing. It goes like this...

Vibration occurs above speeds of 80mph or so. It is felt though the car body, shifter, and clutch pedal. Its quite extreme. You can feel it in the clutch pedal when you just barely have your foot touching it, not depressing it at all. Now here's the kicker - vibration goes away when I push in (disengage) the clutch. Let the clutch out (still at 80+mph) and the vibration returns.

I'm really thinking its not my DS or U-joints or gears or tires, since disengaging the clutch doesn't make them spin any slower. The car has a Pro 5.0 shifter, SLP clutch package, 4.10 gears, stock steel DS, and poly trans mount. That's all the mods that should matter.

Any ideas?

 

Check youre pressure plate does it have any burnt spots I.A.Friction wear

 

That won't cause a vibration.

 

Pushing your clutch in isolates the engine (flywheel) from the clutch/PP and rear drivetrain as long as you're still in gear. So this would point to an engine/flywheel balance issue. If the engine/flywheel were out of balance, it should be rpm dependent, not speed dependent. Try slowly reving up the motor in neutral while parked in your driveway; what do you feel?