Depends what you mean by design...yes, they work the same way, but the one you order for your rear diff is going to be different than the one you order for your front (Let's just go ahead and assume ARB lockers, there's not much else in the way of alternatives for your rig, and it's the best anyway). Both will be actuated by air, and when on will provide constant power to both wheels which is a tremendous traction aid when, for instance one of the wheels has lost contact with the ground. With open diffs, the wheel in the air will get all the power (path of least resistance) and spin while the other wheel just sits there (and so do you). It's amazing how much of a difference even one locker makes to your rig's capability, and allows for much easier clearance of obstacles which with open diffs you might have to rely more heavily on momentum or, gasp, driving skill. Opinions vary on this, but if you're going to have only one (and that's a start) I say lock the rear. The counter-argument is it's easier to pull than to push, etc., etc., I just think rear is a better starting point because your rig behaves much more similarly with the rear locked to what it did when it was open, and you get used to it more easily.
What I've done with my first gen is installed a rear ARB locker, actuated by my powertank, and have allowed for eventual installation of the front locker in my execution of the rear install (Powertank sells a manifold you hook up to the Powertank, and lines run from it to both lockers).
Now, back to your original question, and subsequent things mentioned (lots of highway driving, wheeling not close to your house) I would say definitely do the locker(s) over the SAS, no question. The next wheeling-aid you could do, before a SAS, is crawler gears in your transfer case. With a rear locker and crawler gears, you can get up anything your rig fits over or through. Crawler gears are pricey, but not as pricey as a SAS. Drb's estimate of 3k is the bare minimum. Using the Calmini kit, in addition to what Drb said, you need a donor axle and coilovers. Figure realistically a minimum of 5k.