So just wanted to update, since it's been awhile.

Ended up replacing both UCA's (passenger side upper ball joint blown out), and front wheel bearings with bluepitt Koyo bearings (even though original stock were fine, keep as spares), and also replaced passenger CV axle that was showing some boot separation. Some improvement after UCA's were replaced. However, steering wheel shimmy and subtle vibrations after 65mph remained.

I ended up going back to Firestone and had them swap out the Destination AT2's for Bridgestone Revo 3's under warranty.

I'm happy to report, problem solved.

Shimmy and vibrations are GONE.

Firestone said it was probably only one of the tires that was bad. I call bs, because all four tires were rotated in every which way, rebalanced a half a dozen times and also Road Force balanced, and they would lose balance after a few days driving and start shimmying like crazy again. So either, I was unlucky to have had a bad batch of Destinations installed, or they simply don't balance probably and ride like garbage. Either way, I would suggest avoiding those tires at all cost. I'm finding that the Revo's far out class the Destinations when it comes to absorbing uneven and poor road conditions, so the ride is orders of magnitude smoother in general.

It was a long battle, but I learned a lot about rebuilding the front end and never discount the fact that your tires might be the source of your vibration woes.