My brother and I spent the whole of the last two days try to sort this vibration in his car.
We firstly, replaced the Guibos (rubber donuts - all three), had the tailshaft balanced, re-install to no avail. Vibration would still re-appear at around 3500rpm.
We took the clutch out and replaced it with a brand new unit (clutch needed doing anyways...
Following Haynes manual, Mentions no balancing of clutch unit, as we purchased plate, bearing and pressure plate as a kit Clutch align tool, replace bell housing, bleed clutch
Start vehicle, vibration still lurks as bad as ever.
Is there any other thing we may have overlooked.
I read of guys loosening everything, putting car on jack-stands, start engine, rev change gears and then re-tighten everything with great success. Will try this.