Really easy. But I think I know the cause of the vibration.
The yoke on the drive shaft!!!!
As we installed the new slip yoke, we noticed that the yoke which is welded on the drive shaft was bent.
We corrected it and so the vibration almost disappeared!!!
We must have bent it as we installed a new u joint long time ago…
We once corrected it in the past, but not enough I guess…
