Eh, some are wrong some are right.
In a part-time 4x4 system you only have 2 differentials, which are in the axles. Either one of them can be equipped with a locker. For instance, my Jeep has a stock LSD in the rear. Front drive-shaft speed is the same as rear, Meaning if you have traction on one of the axles you will move. This also means you can't drive it in hard surfaces because of diffrent speeds on the front and rear axle.
In a full-time 4x4 system you have one more differential, which is in the transfer case. If all the diffs are open, all power can be transmitted to it. Some 4x4 systems come with a viscosity-based center diff, electrical or mechanical center diff (systems which transfer a certain procent of the power to the axle which has traction). Some also have a lock-able center diff.