* Driveshaft Balance: Even with new U-joints, the driveshaft itself might be out of balance. This is a common cause of vibrations at highway speeds. A driveshaft shop can diagnose and balance it.
* Differential Problems: Issues within the differential, such as worn bearings, a damaged pinion gear, or even a slightly bent axle, can cause vibrations that become noticeable at higher speeds. This requires a professional inspection.
* Wheel Balance (Despite New Tires/Rims): While you've replaced tires and rims, improper balancing of the new wheels is a strong possibility. Even a slight imbalance can cause noticeable vibrations at higher speeds. Double-check the wheel balancing. Road force balancing (which accounts for tire and rim imperfections) is often recommended for a better result.
* Transmission Issues: While less likely given your statement that it "runs great," problems within the transmission (especially if there's internal wear) could manifest as vibrations at higher speeds under load.
* Axle Shaft Balance: Similar to the driveshaft, the axle shafts themselves could be out of balance.
* Tire/Rim Combination Issues: Even with new tires and rims, there might be an incompatibility between the two, causing vibrations.
Troubleshooting Steps:
1. Wheel Re-balance: This is the cheapest and easiest first step. Get all four wheels road force balanced by a reputable tire shop.
2. Driveshaft Inspection: Have a mechanic inspect the driveshaft for bends, worn components, or any signs of imbalance. Balancing is often the solution.
3. Differential Inspection: This is more involved and requires a professional mechanic to check for bearing wear, gear damage, or other issues within the differential.
4. Transmission Check: Have a mechanic check the transmission fluid level and condition. Listen for unusual noises during operation.
5. Axle Shaft Inspection: Examine the axle shafts for damage or imbalance. This requires professional assessment.
It's unlikely to be a simple fix. Start with the wheel balancing, then proceed to the more complex possibilities with the help of a qualified mechanic. They can diagnose the specific cause accurately. Remember to mention the speed range of the vibration to the mechanic for better diagnosis.
