Mmmm... 4 wheel drive... where to begin...
OK, on any standard drive axle, the power only goes to one wheel. This means that in a 2wd you are only getting power to one wheel. This is most observable on ice or in snow.
4wd gives you another drive axle. But with the new axle you also only get power to one wheel at a time.
Turns out this is the wheel with the least resistance, on that axle.
So, four wheel drive is ACTUALLY two wheel drive.
This becomes important in a situation such as this:
Now there is not much weight on the front driver-side tire, nor the rear passenger side tire. In some situations they will actually be hanging in the air. Since the power goes to only the wheel with the least resistance, the two hanging in the air get the power and you go nowhere.
A similar situation will happen if you pull over to the side of the road in winter, say putting both passenger tires on solid ice, with both drive tires on the pavement. When you try to drive away, the tires on the ice have the least resistance (once they start to spin) and you'll get nowhere fast, despite having 4x4 and two tires on good pavement.
With me so far? :shock:
Now these are extreme situations but they illustrate the point. So what a limited slip application does is give partial power to the other wheel. In other words on the ice or hanging in the air, the other wheel on the axle with the limited slip would get a little of the power. You wouldn't be at a dead standstill, but you can't always move out of an extreme situation like the illustrations.
However in situations like mud and loose dirt, this can provide more power at more places and keep you from getting stuck, or help you get unstuck.
Limited slip doesn't really engage or disengage, but there's a technique using your brakes to make it work better (I've actually gotten out of a twisted no traction situation with my Jeep Wrangler using only limited slip and the brake to prove it would work).
So... that brings us to a locker. A locker basically locks both wheels together so you get FULL power to both wheels. In the tire in the air situation you can drive forward almost normally (which can be bad as you can put the tires in the air even further away from the ground without noticing... it becomes quite like a tetter totter.)
The SMB locker is an ARB which is a top locker that is turned on and off by air pressure. In other words when off you have the equivalent of a limited slip and when on the wheels are locked together. In the ice on the side of the road example you wouldn't even notice there was ice with even one locker engaged.
For loose material, non-extreme situations, like deep mud, getting 100% power to both axle wheels is great.
Unless you're going to do extreme wheeling lockers front and rear are overkill, in my opinion. Getting a locker in back gives you some piece of mind... if you're starting to get stuck or going through something you know will be rough, you turn it on and save yourself worry and trouble.
I think you would be more than adequate skipping the locker. Get stuck six or so times and you can always opt to put one in.