Rear differtionals are used in cars that are rear wheel drive. They are found mostly today n larger cars and most light trucks. A car or a truck that is rear wheel drive will haave leaf spings on either side. Leaf springs are mounted to the car and hld the diffential ot rear to the car. A long metal drive shaft mount to it. The drive shaft is held in place by u joints. One end of the drive shaft mounts into the transmission the other end mounts into the rear or diferential of the vehicle. The differential or rear housing is a large stamped or premade metal housing for that specicic car or truck. Inside of the housing are located 2 drive shafts as well as a set of gears The gears and drive shfts are joined together in the middle of the houing sometimes referred to as the pumpkin, Over them is a removable metal cover held in place by a series of bolts. Some services are neded to be done on rears or differntials in order to keep them working properly. They include intalling a new gasket or material on the plate and cover . Other services can include istalling new ujoint on the front and rear of the drive shafts. Drive shafts can sometime come apart from the gear assembly. If this happenes the car will not move because gears are unable to turn the wheels, At this time it is required to service the rear differential. Either the entrre assembly must be replaced or the drive shafts must be re-installed in the housing. If a new or used unit is to be installed firt remove the leaaf springs and have a jack in place to lower the rear ouit of the vehicle. IOf reinstalling the drive shafts is the way you want to proceed first deterimne that the gears drive shafts and houng are ok to reuse, This invlolves remving thee metal housing and after determing the gears have all of their teeth replacing the drive shafts correcetly. Next intall the gasket or gasket material over the center of the housing. Finally re-install the entire housing and remount the leaf springs. This is a genral overview of how to inmstall and service the rear differentils in most cars and light truck, More specific intructions can bve found in a service maula for that car or online at Alldata or Mitchells.