Doesn't sound like a wheel/tire imbalance, those typically start at higher speeds and get worse the faster you go.

This does sound like out-of-round tires. This typically feels like an odd shimmying at surface street speeds, as if the pavement were slightly wrinkly, but fades away at highway speeds.

If the tires are substantially worn it's probably best just to replace them. Otherwise have them tested on a Hunter roadforce balancer to evaluate the runout in addition to the balance. This will tell you whether one or all are out of round and by how much.

- JimY