The purpose of an Automatic Transmission is similar to that of a manual transmission, the auto tranny's primary duty is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds.
GM Astro/Safari vans were produced with a 4-speed manual or a 5-speed manual transmission. Manual transmissions feature a driver-operated clutch and a movable gear stick which are commonly swapped with automatics in custom applications and transmission conversions.
A differential is a device employing gears capable of transmitting torque and rotation through shafts. Essentially the differential allows each of the driving wheels to rotate at different speeds in the rear axle.