First understand that locking differentials means that both wheels on that axle will both have the same power applied to them and both will spin at the same speed which does not allow the vehicle to steer around a curve properly, vs an open differential or a positraction type differential that allows the wheels to turn at different speeds therefore allowing you to steer the vehicle around a curve.
Auto locking differentials, like a Detroit Locker, are good units if installed in the rear and are used in an offroad vehicle, or a drag racer in the rear. These same auto locking units will get you hurt in Ice and snow because they will lock without warning at a bad time and now you are fishtailing the rear end of the truck. Even worse is having auto locking differentials in the front. If you hit ice and snow in one of these you will not be able to maintain control and off in the ditch you go, plus you cannot steer a vehicle safely on the highway with the front differential locked. A limited slip differential is an option especially in the front but they do not provide the positive power to all the wheels a locking differential provides.
To overcome these problems and still have all 4 wheels providing power in offroad situations but still able to drive the vehicle on the highway you need Selectable Locking Differentials. These are air powered, electric powered or mechanically engaged (cables) and allow the vehicle to travel with open differentials on the highway with the good highway manors that open differentials provide but the user can select to lock one or both of the differentials if needed. Examples would be locking them to get out of a ditch during a snow storm, taking the vehicle thru a muddy field to check on the cows or in any other offroad situation where you need more traction. They are to be used sparingly and only when moving forward is more important than steering.
With both systems installed in the front and rear you have true 4wheel drive where all wheels have power but with the selectable units you control when they engage therefore allow use both on and off the road.
All Wheel Drive is a totally different animal in that the center has a differential that shifts power between the front and rear as needed plus the differentials in the axles have traction devices to allow power to switch between the two wheels on that axle so that the wheels can turn at different speeds which allow them to be used on the highway. Think Subaru, Audi, etc. This system is not considered by many to be true 4wheel drive because at any one time power may not be available to one of the wheels and therefore does not provide the maximum traction you would get with having them locked but you can steer the thing.
Now my 2002 Ford Super duty came with a 4WD system that had a positraction rear differential and an open front differential. So even though it is called 4WD it was at best 3WD when the positraction in the rear worked, which was seldom and not for long (it failed) because the best it would ever do is provide power to 3 of the wheels at a time. I replaced the rear differential with an ARB Air Locking Selectable and removed the open differential in the front and also added a ARB Selectable differential there giving me the option of having all 4 wheels with an equal amount of power at my discretion. So my Super Duty is what many would call a "Real 4WD" vehicle.
Sorry to be so long winded, but there is a lot of confusion with AWD, 4WD, Auto Locking, Selectable Locking, Center Differential, Locking Center Differential, Full time 4WD, Part Time 4WD, and I hope I have not confused things too much.
Excellent post.
