What is the harm of driving with a bad U-joint?
The harm would depend on the nature of the U-joint failure. Like most parts, U-joints can fail in a number of ways. Deterioration of the rollers results in slack. This produces a clunk when moving between gears. The harm is the sudden shock to the driveline when the slack is taken up. Seized rollers may show no slack at all, but make the shaft difficult to rotate. This often shows up as a vibration. The shaking can damage the output area of the transmission and the input area of the rear differential. Seized U-joints can also damage the driveshaft by causing wear in the yokes.
