When there is a leaking differential axle shaft seal in the rear end of your vehicle, gear oil is spilling out into the rear brake assembly hindering the ability of the operator to stop the vehicle. This seal surrounds the rear axle as it exits the rear end in the rear wheel assembly. In time like all seals, the differential axle shaft seal will wear down because of the constant friction between the seal and the axle shaft as the vehicle is being operated. Because it is protected by the wheel hub at the end of the axle, contact with a foreign object is nearly impossible that could cause damage to it.
The differential axle shaft seal is made from a metal ring with a polymer liner. It is this liner that creates a seal in the rear axle without restricting the movement of the axle when the vehicle is being driven. The gear oil in the differential is used to lubricate the rear axle bearing. This bearing is just behind the differential axle shaft seal and must prevent the loss of rear end oil. When this seal wears, the lubrication of the bearing will discontinue because the oil level will drop because of this leak.
To replace a worn differential axle shaft seal requires the vehicle be lifted into the air and the rear wheel taken off. The plate on the differential also has to be removed so the pin or clip holding the axle in the differential can be removed. Then the axle can be pulled out which exposes the differential axle shaft seal. The seal can be pried off with a screwdriver. Be sure not to damage the bearing behind it.