Re: Diff Lock
The diff lock on a standard, factory vehicle works by locking the front and rear drive shafts together. There are three diffs, one on each axle and one between the two drive shafts. Without the diff lock engaged you will be able to loose drive with just one wheel spinning (i.e. on ice or mud), however, with the diff lock engaged you provide constant drive to both the front and rear axles, which means that should one wheel start to spin you will still have drive on the free axle. Should you have two wheels spinning, 1 front and 1 rear, you will still come to a halt, which is when aftermarket cross-axle lockers come in.
One warning though; do not drive great distances with your diff-lock engaged. This creates 'wind up' in the system, which can lead to catastrophic (and expensive) failure. Only use the diff-lock when you think you are likely to loose traction, knowing this comes with experience.
Hope this helps.