- have all wheels balanced off the car
- check the suspension links at the back end, especially the bushes. make sure everything is tight
- check the universal joints in the front to rear (north south)prop shafts
- check engine mounts and gearbox mounts
- check wheel bearing end float at both ends
- chekc to ensure all cyls are firing properly (plugs and injectors)
Thats about all I can think of ATM
I recall someone else having such a problem (or at least similar) on another forum/group. I'll poke around and see what I find