95R
Thanks for the comments.
The rear tires are bigger than the front so it would be difficult to move them. I have switched them side-to-side with no improvement. I have considered borrowing 4 tires and wheels form a friend to see if that helps.
The pinion bearings are new so that is not the problem.
A friend also suggested the tranny may be the problem so I took the car up to the point of vibration and put it in neutral to see if the vibration would stop... no change so it isn't the transmission.
One other possibility is the front U joint. It was difficult to get the clips in so this joint has a bit of stiffness or resistance. I thought that it would "work" itself in but that may not have loosened up.
I plan on removing an axle tonight to measure the bearing. Mustangs Unlimited have more than one size of rear bearing for the 66. Better to measure first then order.
The bearings are relatively cheap so I am going to change them to see what happens especially since one has some play which can't be good.
Wayne