I bet the tires are out-of-round. Your symptoms are typical of such. At highway speeds the tires are acceptably smooth because they are balanced, though a rounder tire will be smoother.

Jack up each corner and spin the wheel/tire on the car. Use a fixed marker (such as a jackstand) placed near the tire to estimate runout. Anything more than 1mm will be a problem.

I had Kuhmos once, that's why I know a bit about these symptoms and causes. Never again.

- JimY