It depends on what you have, If you have a AWD you can change the power in the passenger compartment with a knob or shifter, You just select how you want the gears to engage and where you want the lock to engage either rear wheels or all wheels. You also ones in the cab that will lock the differential in the cab by switching into 4wd automatically and you will hear a click. You also have manual ones where you have to lock the hubs before you can engage 4wd. Just lock the hubs and put it in hi or lo and you are set. Differential also have different gear ratios on all trucks and different lockers. I like Dana 44's personally. There are also 2wd vehicles that have real locking differentials that lock the back tires. The purpose of them is to control power to each wheel. Some will lock all the tires and keep going while others will make the tire spin still when it is off the ground. There is a lot of info out there pick up a Petersen's 4Wheel and Off Road. I love them great info on 4x4ing.
