Hi,
A few things you could check which might cause your vibration.
Wheel balance, has this been done correctly? it's important that the wheel is held on the balancer by the taper as the central flat face is not true.
Prop shaft UJ's are another possibility, check for wear / play and check alignment of the UJ's. Distorted wheels or loose spokes are another possibility, this could be checked when the wheels are on the balancer. Also distorted or bulges in tyres are another. Check the dates on the tyres also.
Just a few quick things to check without getting too involved.
Hope this helps a bit.
Nick