Car wheels on an axle spin at different speeds when turning, each wheel travels a different distance
through the turn, and that the inside wheels travel a shorter distance than the outside wheels. If the
wheels were driven directly on both sides of the axle the inner wheel would have to slip on the road
surface, wearing out the tyre and putting stain on axle components. The diff (differential) allows the wheels to turn
at different speeds with an arrangement of gears in the diff.
WHY IS THIS NOT GOOD FOR A 4X4?
The down side of an open diff is that by its nature, it will allow one wheel to drive and let the other wheel spin, as it always supplys power to the wheel with the least resistance. In an offroad situation, with one wheel in the air, or in slippery mud it will supply power to this wheel, and you will come to a stop! Obviously you will have another axle helping you out with a 4x4, but if one wheel on each axle is slipping you won't get anywhere.
To overcome this you need a Locking or LSD diff.
A Locking diff will apply power equally to both sides of the axle, thus turning the wheel with no traction and the wheel in contact with the ground allowing you to move again.
A LSD is a LIMITED SLIP DIFF. it will not supply exactly 50/50 split to both wheels like a locking diff, but will supply a little power the wheel with traction to help out in slippery situations.
LSD's still allow differential for daily driving, but a Locking diff must be switched off (it will be
Automatic or manual operated) for road use to allow the diff to work properly on hard surfaces.
TRANSFER CASE
Your 4X4 has a TRANSFER CASE behind the gearbox, this is the device that splits the drive from the gearbox to the front and rear axles. There are two kinds of 4x4's...
Part Time 4X4 - For road use these types only drive the rear wheels, saving on effort of turning the front axle when you don't really need it. Going offroad requires you selecting 4WD to supply power to the front axle. You should not use this type of gearbox in 4WD on hard road surface because, like an axle with its diff locked, there is no slipping on the road surface and the front and rear axles will try to turn at different speeds as you turn and cause stress on the drive train components.
Full Time 4X4 - This type of T/Case drives front and rear axles all the time, to allow for the difference in road speeds of the front and rear axles, these T/cases will have a differential built into them. Therefore they must also have a diff lock built into them to allow you to lock the T/Case diff when offroading, with the diff lock engaged they work the same as a part time T/Case that is in 4WD.
The Transfer case will have two gearing options, High range for normal driving and Low range for extra power on steep hills or mud holes. Some T/cases have an Neutral option for running PTO (power take off) devices. (rarely used these days).
Do not confuse the transfer case diff-lock on a full time 4x4 car with an axle diff locks, they are not the same.
OTHER STUFF ON YOUR AXLE.
Front axle drive hubs
There are 3 types of front hubs fitted to 4x4's. On part-time 4WD cars, although the T/Case is in 2WD and not
driving the front axle, the road wheels will still turn all the front axle components while driving along,
(front axles and front differential) To save on wear and tear on these items, and more importantly to improve
MPG the front hubs are disengaged from the axle when in 2WD
Manual Hubs
Fitted to older 4x4's these have a knob built into the front hubs and you have to get out and manually engage
them to get 4WD
Automatic Hubs
In an attempt to save people getting muddy feet, various types of automatic, or electric (switch on dash), hubs
have been tried but tend to be unreliable in practice.
Front axle disconnects
In an other attempt to improve on the above two options, and to allow 'shift-on-the-fly' 4WD engagment,
(the above two you have to stop the car to turn them on and off) systems like Toyotas ADD (automatic disconnect
differential) have a sleeve that joins two parts of one of the axle shafts when you switch to 4WD, allowing shifting
into 4WD at speeds of up to 50mph. In 2WD the wheels spin free (although the side without the sleeve on it still
turns the spider gears inside the front diff, meaning wear and friction isn't quite as reduced as the other two types
of hub)