My 97 has a similar vibration at those same speeds. Over the years I've tried various things to diagnose it, and the best I've come up with is having the tires rotated and balanced, to see if it made a change, and it did. However, within a month or so, if the vibration had gotten better after rotate & balance, it would be just as bad again. Brand new tires made it go away, but after a few months of driving, it is always back.
Tire dealer tells me I need new struts, points to slight cupping wear on my tires. I spoke with a coworker who used to race cars, and he agrees with this assessment.
I've had brakes done with new rotors, no change in vibration, so I suspect this is not a warped rotors issue, although you typically hear from several members who did get vibration because of warped rotors, so it'* definitely something to check.
If you have the luxury of swapping new tires on, that'* a pretty big giveaway if your vibration goes away.
For now, I just drive it as-is, but some time soon I'll probably have struts replaced around the time I'm due for new tires.
97 SE, teflon wiper blades, Dunlop Sport A-2 tires, Leather steering wheel cover, Pioneer DEH-P480MP mp3 CDRW head unit, Pioneer 12-disc changer, iPod hookup, Boston Acoustic 3 way 6x9'*, Boston acoustic separates in door pods, Alpine 4 channel amp in trunk. Magnaflow dual outlet muffler and Ractive stainless tips. Autozone rice pipe and K&N 4750 intake kit. Clear corner lights. Energy Suspension end links.