The issue described occurs at high speeds, specifically above 74 mph, indicating a problem that becomes more pronounced under specific driving conditions. This shaking can be intermittent or consistent once the threshold speed is reached. The severity of the shaking can vary but often leads to discomfort while driving and can potentially affect vehicle control and safety. The fact that the rear tires are worn out while the front tires are new is a significant detail. Uneven tire wear can cause imbalance and misalignment, leading to vibrations felt throughout the vehicle. Additionally, worn tires may have developed flat spots or uneven tread wear, exacerbating the problem.

The likely root cause of the shaking is a combination of tire wear and imbalance, possibly compounded by alignment issues. Tires are meant to be evenly worn and balanced to ensure smooth rotation without causing vibrations. When the rear tires are significantly more worn than the front, it disrupts this balance, leading to shaking at higher speeds. Misalignment can further worsen the issue, as it causes uneven wear and tear on the tires and affects the vehicle's handling. Ignoring this problem can lead to increased tire wear, reduced fuel efficiency, and potentially dangerous driving conditions due to decreased control and increased strain on suspension components.