As posted before:

Automatic hubs actually disengage briefly when the vehicle is in 4WD and you switch directions, i.e. frontwards to backwards, backwards to frontwards. The front wheels have to roll in the new direction a short distance before the hubs will re-engage (about one foot). This means that you're in 2WD until the hubs hook up again. When you get stuck while in 4WD, if you try to rock the vehicle frontwards and backwards to free it, you end up in 2WD for much of the experience, which means you might remain stuck because you don't really have 4WD to pull you out. When the hubs do re-engage, and if the back wheels are being turned by the engine faster than the fronts happen to be rolling, you get a big CLUNK as the hubs kick in again. That's hard on the hubs and front driveline. Manual hubs do not do this, they are either LOCKED or UNLOCKED.