An axle shaft is a narrow piece, like a pole, that connects a wheel to the gears in the transmission of your car or truck. In most FWD cars, the drive axle is split between two half axles, with differential and universal joints, or u-joints, in between. The constant velocity (CV) joint is a third piece that allows the wheels to move freely. This joint enables the shaft to rotate, transmitting power at a constant speed without causing an excess of friction and heat. When a boot, or grease container, leaks and the cv joint fails to be properly lubricated, it may eventually wear out and require a shaft assembly replacement.