The driveshaft universal joint (U-joint) is a cross-shaped bearing coupling. It sits at the ends of the driveshaft. As the suspension moves up and down, the U-joint allows the shaft to bend while rotating. This prevents the metal shafts from snapping under load.
A U-joint fails when the grease inside dries out. Rust forms inside the bearing cups, grinding the needle bearings to dust. A squeaking noise that matches vehicle speed is the first sign of this rust.