I've just bought a 4.6is 2003 end of March, and I've got the same vibration happening at anything from 20-60mph, the dealer has done a software re-boot but problem still exists, it's in again tomorrow, I'll post how they get on. Also got an issue with occasional very lumpy running (feels like engine is missing badly) this happens at low speed and when engine comes under load, eg. going up hill, it may happen for a split second (which feels like you've gone over a bump) or can continue for several seconds in which case car almost kangeroo's. Again they will look at this again but if anyone has any ideas, it would be good to point them in the right direction.
Thanks.