Thank you Gstguy97. I found the problem to be 3 Michelin tires being
out-of-round. This person who owns a body shop told me about the tires before he even took them out to put on his balancing machine. He has a Porsche 911 in which he purchase 4 Pirellis for. After the installation, he was
getting vibrations at low and especially at high speeds. Two tire shops inspected and balanced his tires but could not find the problem. He then brought it to a third shop where the owner identified the problem immediately.
He removed the tire+wheel, put it on the balancer, and just spun it without the safety cover of the machine. By just looking and focusing on the top of the tire while it was spinning, they saw that it was undulating up and down, meaning that either rim or tire was not completely round. They spun it a second time and focused on the edge of the wheel to ensure that it was round.
Sure enough, his tires were out-of-round. Not all tires were manufactured equally, even within the same make and model.
Consequently, the 911 owner bought a different set of tires - Bridgestones I think, and the vibrations were gone.

So, for you IS'ers out there w/ vibrations; it just might be something quite
simple and unexpected -- your tires.

Thanks.

J