This is simple, but worth looking at. Make sure the face of the rims and the face of the hubs are both squeeky clean with no rust. Even a very small pebble stuck between the faces may cause vibration at that speed. Also, if you have any of those retainer rings on your lugs, ensure they're there on all 4 or not there at all. In fact, if you have all 4 consider removing them.
Good luck and let us know if you resolve it.
