From Faq
Factory equipped TRD Off road package Tacomas come with an Electronic Locking Differential- Diff lock for short. This feature is almost must for anyone looking to buy a new truck. To understand what a differential locker does, you need to understand what a differential is: Think of your truck when it is turning.
One tire has to rotate separately of the other to cover the same distance. The outside wheel has to travel a much greater distance, and therefore has more resistance. A differential allows the wheels to spin separately of each other due to traction differences. (In the above illustration, the outside tire has more traction or- resistance).
This is great for every day on road driving, but once you hit the dirt, you need all the traction you can get. Let’s think of the same open differential: you are climbing a hill, and a wheel comes off the ground. Which wheel has more traction? The one on the ground of course: it has something stopping it from rotating (more resistance) while the wheel off the ground has none. The wheel with least resistance will spin, leaving you unable to continue forward. Simply put, a differential transfers torque to the wheel with least resistance (in this case the wheel in the air, and in the first illustration, the inner wheel).
A differential locker mechanically locks the two wheels together- torque is applied to both tires, regardless of available traction. So both your tire on the ground, and tire in the air will spin. This is obviously a very helpful item offroad. Even though a tire might be in contact with the ground, or rock, it may not have enough weight on it to get enough resistance.
Refer to the first illustration: when locked both wheels are forced to spin at the same speed, one wheel has to lose traction to complete a turn on a high traction surface such as pavement. This is why you will hear your tires chirping and binding if you lock on pavement. This causes a high amount of stress on the axle shafts, and other drive train components. It is for this reason, that you should avoid engaging a locker on pavement, unless you are going to using it in a straight line (ie, to pull someone out a ditch, or similar circumstance).
What types of lockers are there? There is basically two types of lockers: selectable and non-selectable. Selectable lockers will act as an open differential when not engaged, and locked with the press of a button. These selectable lockers include the TRD electric locker and the ARB air locker. Non-selectable lockers are locked full time, although some allow for a small amount of slip before they will "lock". Non-selectable lockers include: Lockright, Aussie, and Full Spools.
What is the difference between a locker and limited slip? A limited slip differential uses clutch packs to apply torque to each wheel, no matter what resistance the wheel will have. Because they are clutch packs, means the wheel with least resistance will still spin, but hopefully enough torque is still being applied to the wheel with greater resistance. The effectiveness of an LSD is determined by the clutch packs- if they are loose, no torque transfer will occur, and the LSD will be very useless. If the clutch packs are tighter, they will transfer more torque to both wheels, making the LSD more effective.
How can I tell if I have a locker? It depends- If you have a selectable locker there will be a button somewhere to engage the locker. However if you're not sure (ie, don't believe the engage switch, or think you may have a non-selectable locker); The method is: lift the axle in question off the ground with a jack, so that both tires are in the air. With the parking break off, and transmission in neutral (Obviously with the remaining wheels chalked on a level surface), spin one tire. If the opposite tire moves in the same direction, you have a locker, if the opposite tire moves in the opposite direction, you have an open differential.
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