A Chevy rear end or differential is filled through a plug. There is no provision for draining it, however, other than removing the rear cover. If it requires differential oil and the rear cover had not been removed for service, there is obviously a leak in a seal. At the same time, the fluid does not get contaminated and should not need to be changed under normal circumstances. There are two instances requiring replacement of the oil: if the differential happened to be submerged under water or a bearing failure has occurred.
Raise the vehicle with the floor jack. Place the jack stands under the axle tubes and lower the vehicle down onto the jack stands. Place rags under the differential.
Remove the filler plug on the driver's side top-front of the differential, using the appropriately-sized wrench. Get the plug ready so that it can quickly be installed when the oil reaches the proper level.
Fill the differential through the plug hole until the level of oil is level with the hole and begins to run back out as it is being filled. The oil containers have tapered necks so they fit into the hole in the differential. Finally, tighten the plug with the same wrench.
Lift the vehicle, and remove the jack stands. Lower the vehicle to the ground. Check for leaks.
