With lockers you simply lock each half of the axle together. Each wheel has a axle shaft that meets at the differential. A locker goes in either the front or rear differential and locks the two axle shafts together so that each axle shaft will rotate in sync with the opposite side. You aren't locking it into 4wd, that would be the transfer case, but you are ensuring an even split of torque to each wheel on the axle.

A 12 bolt is our rear axle, a 10 bolt is our front axle. The 12 bolt is a soild axle, the 10 bolt is a IFS (independent front suspension) axle. Basically, the 12 bolt rear axle has a solid tube that houses the axle shafts. The 10 bolt uses CV axles to join the axle tube to the wheels. This allows each side to flex independent of the other wheel. Better on road ride, but weaker joints. Also, the parts of a 12 bolt axle are heavier duty than those of the 10 bolt. It is typical for the rear axle to be a heavier duty axle in vehicles beacuse it can cary the load of cargo, towing, and is usaully the main drive source.