This keeps up we'll have an entire post-season roster.
Mine also has that vibration, even after balance, alignment, new bearings. I just figured it was my crappy General Grabber SP tires, which are a little feathered due to running some bad toe-in for a while. They sure are noisy as heck.
I've got 65k on my '96, btw.
I avoided the vibration on my way to FL and back recently by setting the cruise on 78mph. Fiddling with air pressure helped, too. My tires were at 28psi after the weather cooled a bit, and bringing them up to 32 smoothed things a bit.
BTW, this is pretty common on Miatas, too, my other cars. It's known as the "65mph Shimmy." Usually, careful attention to balance, air pressure, and alignment can clear it up, but on some cars new tires are needed. It's not generally thought of as a problem with the other suspension bits, as brand-new cars can exhibit it.