an update on my problem, my mechanic got the tailshaft balanced to see if that will resolve the problem.
They said it was slightly unbalanced and re-balanced it.
The vibration has mostly gone away, not as bad as before, but its still present at 100km/h+
My mechanic suspects its the tyres, will try swapping with another set of wheels to see if this fixes the problem.
However, if the vibration was slightly fixed with the tailshaft balance, doesnt that mean there is more of a chance of vibration in the drivetrain rather than the tyres causing the problem?
cheers