Weird Vibration at 2500 RPM's and above

I'm inclined to think the input/output bearings are worn, &/or the pilot bushing. You clearly are certain it is eng. RPM related, "no matter which gear" means it is not the prop-
shaft or output shaft, nor the rearend. A bad clutch disc hub could cause it, but it would still do it in neutral, unlikely it would gyroscopically "center" itself every time it was
engaged in neutral....not impossible but............. I have seen accidental downshifts to too low a gear bend the pressure plate torque straps( they're meant to be pulled on
not pushed on), when they fold up the plate is not centered anymore, but that would not go away as per above. What does simply pushing on the clutch at speed do, &
have you attempted to depress the clutch & maintain the RPM's?