[DonC]

Were you getting the vibration when coasting with the clutch in?
My first thought would be either the drive shaft or (if you have independant rear suspension) the half shafts. Check the flange bolts and also, if it's been balanced with weights, check drive shaft to make sure you haven't thrown a weight.
Ditto the wheels. I'd also look at the side pipe mountings.
Is the rear end still securely bolted down?
A lot of potential areas to look at and not a lot of information from you at this point. Just throwing stuff out there.
DonC