* Driveshaft Balance: This is the most likely cause. Even if the U-joints are perfectly installed, the driveshaft might be out of balance. Replacing the U-joints can subtly alter the weight distribution, causing vibrations at higher speeds. The driveshaft needs to be rebalanced after any major work on it, including U-joint replacement.
* U-Joint Installation Issues: Though less likely given the speed-dependent shaking, there's a chance the new U-joints aren't perfectly seated or aligned. A slight misalignment can cause vibrations, especially at higher speeds. Double-check the installation to ensure everything is correctly aligned and torqued.
* Wheel Balance: Unbalanced wheels can cause shaking at higher speeds. Check your wheel balance after the U-joint replacement, as this is a common source of vibration.
* Tire Problems: Worn or damaged tires can also cause vibrations, especially at higher speeds. Inspect your tires for uneven wear, bulges, or other damage.
* Other Drivetrain Components: Issues with the differential, axles, or transmission could also contribute to vibrations. These are less likely if the problem only started after the U-joint work, but they shouldn't be entirely ruled out.
* Brake Issues: While less probable, warped brake rotors or a seized caliper can cause vibrations.
Troubleshooting Steps:
1. Re-inspect U-joint installation: Make absolutely sure the joints are properly installed, lubricated, and aligned.
2. Have the driveshaft balanced: This is the most likely solution. Take it to a reputable shop specializing in driveshaft repair.
3. Check wheel balance and tire condition: Get your wheels balanced and inspect your tires.
4. Inspect other drivetrain components: Look for any obvious damage or wear to the differential, axles, or transmission. This might require a professional inspection.
In short, while the U-joints *could* be a contributing factor (even if improperly installed), the speed-dependent nature of the vibration strongly suggests a balance issue, most likely with the driveshaft. Have it professionally balanced to resolve the shaking.
