I have exactly the same problem as you described, including bearing noise. Can not cruise in 5th gear under 70km/h !!! It just so happens that my car came with several NISMO goodies (indicators, intercooler, LED tail lights etc.) so I was wondering if the clutch was NISMO as well because surely not all R34s have this vibration problem so I figured the clutch is different...and from what you've said it's the only common thing I can find between us, so this would suggest A) I probably do have a NISMO clutch (yay) and B) it probably is the clutch causing the vibrations...