An axle shaft is the final link in the drivetrain. It connects the differential side gears to the wheel hubs. In front-wheel-drive cars, these are CV axle assemblies featuring flexible joints on each end. In solid rear axle trucks, these are solid steel shafts that slide inside the axle housing tubes. They must handle the full engine torque and bend with suspension movements.
Axle shafts fail due to impact damage or rubber boot tears. If the protective boot rips on a CV axle, the grease escapes and dirt enters, destroying the joints. You will hear a loud clicking sound when turning. Solid rear axle shafts can bend if you hit a curb hard, leading to wheel wobble, tire wear, and differential seal leaks.