It can be found on trucks from 1984-1988, and on 4Runners from 1984-1989. Do not confuse this with ADD. AUTO hubs are NOT similar to ADD. The AUTO hub has a clutch inside that will lock the hub when it receives forward movement of the axle. Since the axles are not always spinning with this design, you must come to a COMPLETE stop before shifting into 4WD. Under no circumstances can you shift into 4WD while in motion with the AUTO hubs.

The AUTO hub will unlock after 1/4 turn of the axle in reverse. This is the major flaw to the design. Often on the trail we need to back up in 4WD. Especially if the vehicle is stuck, and we're trying to free it by backing out of the hole it is in. With the AUTO hubs, once you shift into reverse and begin to turn the driveline the hubs will unlock, and you'll be left with only 2WD in reverse. The will eventually lock into 4WD again, but having them unlock on you at all is very troublesome.

AUTO hubs are notoriously unreliable. The springs inside the hub often break, and the hub will no longer engage. Repairing these hubs is VERY expensive, and replacing them with new AUTO hubs is out of the question. New AUTO hubs are priced in the hundreds of dollars)

The best thing you can do is to replace your AUTO hubs with manual locking hubs. You will increase the reliability of your 4WD system many times over by getting rid of the AUTO hubs, and you will gain the ability to shift-on-the-fly, assuming you're driving with the manual hubs LOCKED.