75mph vibration

Have a look at the new tires balanced. The tires have red and yellow dots on the sidewall when new to help effectively balance the tire. I often see these dots anywhere but where they should be, with lotsa excessive weight added. You may have to look on the inside of the tire, cuz if the shop is respectable, they shoulda washed the dots off the outside. If you can locate either dot, it should be very close to the valve stem..

Regardless, I'd explain the problem to the tireshop, and ask if they could check the balance.. Most shops will do this no charge. I would.

It's also possible the wheel threw a weight causing an imbalance.

hope it helps