Its a resonance issue, the vibrations augment each other at that speed on your X.

If you go up to 110 mph, the vibration goes away.

And, you have a governer on the speedo that kicks in about then, limiting the mph you can go, as the tires are not rated to go that fast, and Nissan hates if people blow their tires out, roll over a few times, and then sue them, etc.