Before we go to far it is important to understand what a differential is and how it works. A differential is a set of gears in the center of an axle that allow each axle half to rotate at different speeds. When a vehicle goes around a corner the inside wheel rotates slower than the outside wheel. A differential allows this otherwise one tire would end up dragging. It is also important to note that a differential will provide up to 100% of the power to the wheel with the least resistance so if one wheel is spinning the other wheel won't turn at all.
Lockers and Limited Slip
A locker provides the ability to lock the differential so both wheels turn at the same speed. This allows you to overcome the problem of one wheel loosing traction as mentioned above. A limited slip differential uses an internal mechanism like a clutch to essentially lock the differential automatically when slipping occurs.
4 Wheel Drive
A standard 4 wheel drive system like my Tacoma's have two differentials. One in the front and one in the rear. When in two wheel drive all the power goes through the rear differential. When in 4 wheel drive the transfer case connects the front and rear differentials providing a 50/50 split of power to the two differentials. When in four wheel drive the front and rear differentials are locked together and must rotate at the same speeds so it is not safe to operate in 4wd while on a hard surface.
All Wheel Drive
In an all wheel drive vehicle the transfer case is also a differential and can vary the amount of power to the front and rear. This allows the vehicle to have torque at all four wheels even on hard surfaces since the 3 differentials can vary the power to all 4 wheels.
Locking Center Differential
The 100 series Land Cruiser is an all wheel drive vehicle that allows you to lock the center differential. Once locked the Land Cruiser's center differential is locked it is in true 4 wheel drive just like the Tacoma mentioned above. Many AWD SUVs do not have the ability to lock the center diff. Some early 100s had a rear locker as well. Newer models have ATRAC mentioned below.
4 High allows you to drive at speed but you should probably not be going very fast. Let's say under 50 mph. 4 high is generally good for slippery conditions when additional traction is needed like in snow, ice, muddy roads, rocky roads, etc.
4L Low Range
4 Low provides significantly more torque through gearing. This can only be used at very low speeds under 25 mph. The gearing will top out the vehicle very quickly. Imagine being on a mountain bike in the lowest gear, you spin the pedals very fast and get up the hill easy but can't go very fast, same mechanical concept. I use 4 low whenever I am going very slow on slippery surfaces, very steep terrain, deep snow, thick mud, sand, deep water, etc. I especially like using lower transmission gears (1 or 2) in 4 low when declining very steep hills and let the vehicle crawl down the hill without the need to do much braking.
A unique feature in the Land Cruiser is the "PWR" button. In this mode the transmission shifts at higher RPMs. It keeps the vehicle from shifting up and down on steep hills and when towing. Some people like it for performance but it will cost you in MPG!
Another unique feature of the Land Cruiser is the "2nd" button. This has the vehicle start in 2nd gear. The 100s first gear is especially torquey especially in the 5 speed. This can cause spinning very easily in extremely slippery conditions. Use the second button when it's extremely slippery and spinning is an issue like ice, sand, etc.
Newer 100 series have ATRAC or advanced traction control. This system recognizes wheel spin and brakes the spinning wheel to allow the differential to send power to the wheel with grip. If you are in a situation where you are loosing grip on a wheel up front, in the back, or both, as long as you keep your RPMs up, the ATRAC will pulse the brakes on the slipping wheels and allow you to crawl out of the situation.