Youtube is your friend these days, 4xoverland has some good how to videos, as does Ronnie Dahl.
Only lock the diff in slippery conditions, i.e. off road. Using the diff lock on tarmac will shag your transmission.
I'll try and give a brief explanation of how the difflock works.
When a vehicle goes round a bend, the inside wheels describe a different arc to the outside wheels.
to get around that, a rear wheel drive has a differential, this allows the wheels to travel at different sspeeds. An unfortunate side effect of this though is that if one wheel starts to spin, all the power is transmitted to the spinning wheel, so you get stuck.
Here's a video
Also, the front and rear wheels describe a different radius of curvature. This is of course not a problem in a 2 wheel drive vehicle. However, in a fulltime four wheel drive (4WD) like yoours that would be an issue because the front and rear axles would be trying to travel at the same speed. In a part time 4wd this is overcome by using a lever, or hydraulics, to mechanically disconnect the front and rear drive, so power only goes to the rear axle unless 4wd is engaged.
In a fulltime 4wd like yours the solution is to fit a differential (a bit like the one in the axle described above) between the front and rear axles. This effectively locks the 2 axles together, which is why you musn't use it on a grippy surface. It is a common schoolboy error to think that to get a fulltime 4wd stuck, you need all four wheels to be spinning, this is not the case. As we can see from the above, without the differential locked you would only need one wheel spinning, either a front or a rear. If you lock the centre diff, then you would need a front and a rear wheel spinning to get stuck (most often seen in a situation called being 'cross axled')
The traditional way around this was to fit locking differentials to the rear, and sometimes the front, axles (when a vehicle has front rear and centre locking differentials fitted it is often referred to as 'triple locked'). This would ensure that in order to be stuck through loss of traction, all four wheels would have to be spinning.
A more modern solution to this is to use the traction control system to send power to the wheel with grip by utilising the ABS. It's a bit like the old school solution which was to lightly apply the footbrake so there was some resistance on the spinning wheel and some power would go to the other wheel. Arguably these electronic systems don't work as well as proper locking diffs, but they vary widely between vehicles and can be very good.
I would disagree with the above comments. Do not wait until you are stuck! read the terrain ahead and select the appropriate vehicle setup for what is coming up. If you wait until you are stuck the vehicle and tyres have to work that much harder to get you unstuck rather than powering through, and that maybe the difference between getting through or not. I often engage my rear difflock when I probably don't need it, because why wouldn't you? I would rather have more traction than less. Just don't do it where there is a lot of grip.
using difflocks on a hard surface leads to a condition called transmision wind up. As we have seen above, having a diff locked means a particula set of wheels will travel at the same speed, when you turn a corner the wheels need to travel at different speeds, but locking the diff means they are trying to travel at the same speed. This isn't a problem on a loose surface, the extra rotation just spins out through the wheels, but if you are in high grip, say on tarmac (even wet tarmac, or even hard packed dirt) the wheels can't spin it out and it will put a big strain on your transmission, eventually leading to failure somewhere in the drivetrain.
Another video!
In terms of high and low range, low range is just a reduction gearbox so you will go considerably slower in a given gear for a given amount of revs. There are a number of advantages to this, such as better control off road,being able to go slowly without riding the clutch, control on steep ascents and decents (which can be extremely dangerous if you don't know what you're doing so research this before it happens) and in terms of the amount of 'torque' available. It doesn't actually increase torque but I can't think of a better way to describe it but lower gears equals more pulling ability.
You might also find this video useful
Crawl control is a new fangled feature
Heres another vid!!!