With two axles turning BFGs, there is great traction potential.  But what if irregular terrain leaves a tire in the air or on a poor traction surface.   With open differentials, power goes to the wheel with the least traction.    So one tire on each axle may spin helplessly despite having 4WD engaged.   To get "true" 4WD the differentials must be locked.  They can't be locked all the time or axles would break when turning corners on clean dry pavement or broad smooth rock surfaces.

4WD vehicle manufacturers sometimes offer a limited slip differential option.  These use clutch plates to lock the differential.  And when turning on dry pavement they just slip.  Unfortunately, they also slip when there is a significant difference in traction between two wheels in an off-highway situation.

There are a number of "automatic lockers" too.  These are supposed to lock when going straight ahead and in off-highway situations but release to turn corners on dry pavement without breaking axles.  However, as anyone who has had one of these installed in their front axle knows, they don't always release when you'd like them too - making it almost impossible to turn.  They are not recommended for driving on icy roads either.  And they decide when the differentials lock and when they don't lock - not you.  If you want to unlock them to avoid slipping sideways on a slick off camber situation - you can't.

ARB Air Locker equipped differentials lock and unlock with the touch of a dash mounted button.  YOU get control over when your differentials are locked and when they aren't.  Slick off camber situation - turn them off.  Extreme irregular terrain that may leave a tire biting the air - turn them on.  Driving on dry pavement - turn them off.  Most users leave them off most of the time for ease of steering and to reduce wear on drive train components and turn them on when needed.

The ARBs are locked by air pressure.  The ARBs are released by releasing the air pressure.   I also use the ARB air compressor to air tires back up after a day on the 4WD trails.

Zig's 4 Wheel Drive in Grand Island, Nebraska  installed the ARB air lockers and compressor for me.  They also took some pictures of the installation:
 

TJ Dana 30 front differential with ARB Locker installed

TJ Dana 44 rear differential with ARB Locker installed

The ARB air line goes into the right bearing cap in both differentials.   Compressed air enters the cap and forces a fork into the spider gears inside the case and locks them until the compressed air is released.

I used braided stainless steel wrapped air line rather than the small diameter blue plastic line that ARB supplies.  The plastic line works very well if properly routed.  And it is easy and inexpensive to repair.  But I had problems finding a location far enough away from engine/exhaust heat in the last installation.   The heat softened the plastic and plastic air line blew out.  The heavy duty braided stainless steel air line avoids that problem.

The ARB Air Compressor was mounted on a shelf at the back driver's corner of the engine compartment, under the back of the speed control.  The quick connect for an air hose to fill the tires is sticking up in the foreground.

It is important to place the ARB Air Locker switches where they can be easily reached.  You'll often be reaching for them while preoccupied by other chores such as keeping your 4WD from falling off a ledge.  The three switch blanks in the center panel just above the cigarette lighter/accessory pug worked out perfectly.  They are in easy reach and the installation leaves a very clean interior with no clumsy switch plates hanging down at odd locations.  From left to right is a rocker switch to turn the compressor on and off, the ARB front differential locking switch and the ARB rear differential Locking switch.   All the switches are lighted when on.

The microphone is actually a Cobra 75 WX ST CB radio.  It is a full power unit with SoundTracker technology.  The entire radio and all controls are in the microphone.  It includes a lock button to avoid accidentally changing channels while handling the microphone. There is a small junction box under the dash from which the antenna and power cabling is routed to their sources.

ARBs served me well in my previous 4WD.  Lockers are so effective that serious 4WD events require them in both the front and rear differentials for their most difficult trails.  The ARBs allow you to choose when they are on and when they aren't with the touch of a button, providing significant advantages over 4Xs with "automatic" lockers.