I too am looking into a locker for my pile (in the thread below this one) but i can tell ya what i know.
Basically a locker \"locks\" both axle shafts/wheels together on one axle. On most vehicles if you hit the gas one rear tire will spin, usually the one with the LEAST traction (this is an open differential), some vehicles when you hit the gas the rear tires will take turns spinning, or both will spin but with less power (this is a limited slip or posi differential).
Your jeep is actually 2 wheel drive, it\'s just that you have one front and one rear wheel gripping (when in 4wd), if you put a locker in both axles then you will be 4 wheel drive
With a locker both rear wheels will spin together at full power, giving you the most traction. Now i said rear but you can put a locker in the front axle too. Most will say to put a locker in the front cause it helps the most by pulling you over obstacles.
Now there are several types of lockers that work in different ways. I am looking at an Aussie locker, they are like an \"auto\" locker and lock when the one wheel starts slipping. They usually work with gears and springs or clutches to \"lock\" both axle shafts together allowing both rear tires to grip.
You can also get ones that lock via a cable, electicity or air pressure. These ones are usually quite a bit more money, plus you have to engage them for em to work.
Honestly you probably don\'t \"need\" one, i have been runnin for 3 years without one....now i am not a hardcore off-roader/rock climber either, and my junk does pretty well for the areas i play in. It actually has suprised me how well it gets around with no lockers.
I would say get a small lift, like a spacer lift, and get some aggresive tires and try that out I am positive you will be VERY suprised at what these thing will do \"right out of the box\" they a capable and fun, i have a small lift and good tires and it is really teaching me how to drive off-road
If you want one and want to get it installed Ryan_25 on here is prolly the guy to talk to he has a shop in Farmington i think.
Sorry for the book and hopefully it helps ya some, if i said something wrong i am sure someone will correct me, as i too am tryin to figure this all out.
my .02, Jason
Aussies website
http://www.aussielocker.com/you can read this too for some info
http://en.wikipedia.org/wiki/Locking_differential 
