Four Wheel Drive

The most common four wheel drive vehicles work in similar ways. They have a transfer case that can engage the front wheels and lower the rotation speed in a low drive mode.

2H - 2 high

In 2 high mode the front axle spins freely while full power is sent to the rear wheels. This is usually used for the highway and gives better fuel economy.

4H - 4 high

In 4 high mode the front and rear axles spin at equal speed and power is distributed between the axels. On most 4x4's the front and rear drive shafts lock together, but on some 4x4's especially those without 2 high mode you have the option of manually locking the center differential unit (diff lock). Using 4 high mode gives your truck the awesome traction needed for the off road. It is absolutely essential to use the 4 high mode only on slippery surfaces like snow and ice, gravel or other rugged terrain.
Shifting into 4 high while driving is usually possible but you should keep a low constant speed and not engage it while the driveline is under strain.

4L - 4 low

In 4 low mode the 4x4 is ready for extreme off-road situations, like rock-crawling and heavy snow or sand. The lower differential gear ratio gives greatly increased power at the slow speed needed.

Locking hubs

Some 4x4's come with locking hubs for the front axels. The manual type is usually much more robust and is preferred over the automatic one. If the truck is equipped with locking hubs they must be in the LOCK position to enable four wheeling. The FREE position can reduce wear on the differentials and can also come in handy if you break the front drive.

Lockers

The wheels do not all spin at the same speed. When turning the inner wheels travel a shorter distance and must therefore turn slower, the rear wheels similarly turn slower. Differentials are used for applying the same torque to each wheel.
Standard 4WD sends power to the wheel that has the least traction. If for example one wheel is on ice and the other is on sturdy ground, that will be the wheel rotated! That way you might not move at all.
In order to spin all tires equally at all times it is advisable to consider installing lockers for the front and rear differentials. The most common ones are Detroit Locker, Lock Right and ARB air lockers.
Lockers are the secret ingredient to successful off-roading.
When you have lockers in your front drive, steering can be affected when the lockers are active. In fact for a 100% locked front drive turning on hard surface is practically impossible since all wheels are forced to turn at the same speed.