A U joint, or universal joint, is a type of connection used in many applications, but primarily in a car or truck's drivetrain. The universal joint takes its name from the ability of the joint to move in any direction.
Most rear-wheel drive and four-wheel drive trucks have two universal joints on each end of the driveshaft. The driveshaft runs from the transmission to the rear differential and works to convey power from the engine to the wheels.
The universal joint itself is cross-shaped, but when connected, the joint is identified by the U-shaped end of the driveshaft where it connects to the U-shaped end of the transmission and rear differential.
As a universal joint wears out, it often will make a metallic thump when the truck is placed into drive or reverse. This is due to the driveshaft taking up the slack in the joint. Worn universal joints also may cause the driveshaft to spin out of balance. An out of balance driveshaft may vibrate when the transmission is engaged.
