The front wheels shimmy or bounce above 40 mph, even on a smooth, straight road. The likely cause of this problem is:

When front wheels shimmy or bounce at speeds above 40 mph on smooth, straight roads, the likely cause is often related to the balance of the tires. When tires are out of balance, it creates uneven weight distribution while the vehicle is in motion. This imbalance can lead to vibrations that manifest as a shimmy in the steering wheel or suspension system, particularly noticeable at higher speeds. Since this issue tends to become less pronounced at lower speeds or on uneven road surfaces, it aligns with the scenario presented in the question.

In contrast, brake issues typically cause problems like pulling to one side or grinding noises when brakes are applied rather than shimmying at consistent speeds. Worn suspension parts can lead to a host of other issues, such as poor handling or excessive body roll, but wouldn't specifically cause a shimmy in the wheels in a direct manner, especially on a smooth road. Misalignment can affect the vehicle's handling and lead to uneven tire wear, but the characteristic bouncing or shimmy above a certain speed points more towards an imbalance in the tires rather than alignment issues. This makes out-of-balance tires the most fitting explanation for the symptoms described.