I expect the problem is wheel balance and/or motor mount related.
I know you said the wheels have been rebalanced, but it is extremely common for them to use weights only on the inboard edge of the wheel. This is called static balancing and will NOT correct vibrartion, it only keeps the wheel from bouncing up and down. If you balance the wheels on a properly working computer balancer on both planes(inboard and outboard) you will keep the wheel from "wobbling." This is an extremely common problem because there are many tire stores that think that they only need to static balance wheels. I got so tired of fighting with them that I bought my own Coats 1001 computer balancer. Every car I have can now be driven at its top speed with ZERO vibration.
To the motor mounts, when one goes bad it will make you think you're in a B17.