Selectable lockers are often used in the front, that's true. But that's mostly because it's so nasty driving an auto locker in the front that people will pay more for a selectable there, while they will more often choose to save a couple hundred buck and run an automatic locker in the rear.
I have heard one person claim that the ARB is junk, and if it is marginal I could see it being more likely to break in a rear axle. But most people seem to think ARBs are good quality units, and if most people are right, then it really shouldn't be an issue.
Are selectable lockers easy to install? The simple answer is "no." It requires setting up the ring and pinion gears. That's not rocket science, but it's one of those "if you have to ask you probably can't do it" sort of things. If you want to do it yourself, look up instructions on-line. You can decide if you're willing to buy the tools required and if you have the patience and skills required. Otherwise you need to take it in to a shop.