ROBERT BONNER
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Since the vibration is gone for now, imbalance can be ruled out as a root cause. While manufacturing defects and damage are exceedingly rare possible root causes for driveshaft U-joint failure; the typical root cause is misalignment of the components at the opposing ends of the driveshaft (differential and transmission for 2wd/transfer case for 4wd). The two have to be parallel with very little tolerance for deviation. This is a very difficult thing to check in the vehicle, though not impossible. If the vibration returns, even a little bit, I would take it back and have them check the driveshaft alignment for parallelism.
The vibration should initially present as a very high frequency (for a vehicle) at road speed. The 10th gear is overdriven nearly 36%; so, at 80 mph/2000 rpm, the vibration will likely cycle at over 50 hz. It's like a bass frequency hum. By the time yours was replaced, it probably presented a lot more "music"; but, at the beginning of the failure, it will just hum at higher speeds.
The vibration should initially present as a very high frequency (for a vehicle) at road speed. The 10th gear is overdriven nearly 36%; so, at 80 mph/2000 rpm, the vibration will likely cycle at over 50 hz. It's like a bass frequency hum. By the time yours was replaced, it probably presented a lot more "music"; but, at the beginning of the failure, it will just hum at higher speeds.