kvo and trckmagik give good advice. I think you would be happiest with a "lunchtime" locker, which is a locker that installs in your present differential carrier. They're less expensive and fairly straightforward to install. I've personally never used one, but heard many positive comments regarding them. The Powertrax and Detroit Gearless are the two I would consider, I've heard they work well for most and are fairly seemless in operation.
The ARB air locker is a good choice, but quite a bit more expensive and involved.
I've run an Auburn limited slip in my rig and had good success with it. It provides a high level of traction and is seemless on the street. It is however more expensive than a Powertrax or Gearless to install though, since it replaces the stock carrier, which means that you would have to have your gears reset upon installation, which really requires quite a bit of experience to do it right. Like I said, it's good, but then again, it's not a locker. There will be times when you will overpower the bias of the unit, which means you will get wheelspin. For most mild to moderate situations though, it works well.
I currently have a Detroit locker in the rear of my rig. I love it. It's heavy duty (you'll probably break something else before it breaks). It is expensive though, and it also replaces the stock carrier. In your situation, I don't think this would fit the bill.
Any automatic locker, whether Powertrax, Gearless, Detroit, etc., will lock both wheels together when torque is applied. So anytime your wheels are under power (whenever you step on the gas), both rear wheels will be locked together, whether you are going straight or turning. When power is NOT applied, the device will allow differentiation, meaning both wheels can turn independently of each other. This is useful when you're in a low speed, power off turn, like in a parking lot or such. When power is applied again, both wheels will lock together again. This is the part that many who are inexperienced with lockers don't like. Some lockers can be noisy when they engage, and the vehicle will all of a sudden seem like it wants to go straight, because both wheels are now wanting to turn at the same speed. I've heard that the Powertrax and Gearless have less of these undesireable road manners when they engage. With my Detroit, every once in a while there will be a clunk when the wheels are locking up, and since the vehicle is now trying to go straight, a little wiggle is felt. I'm used to it now, but if you've never felt it before, it can be somewhat unsettling to some. With any locker, the "clunk" does not hurt anything, it's just part of the operation of unit.
The ideal thing for you would be to drive someone's rig that has a locker in it, so you would know what it feels like first hand. I know that might not be possible, so in that case you just have to rely on people that can steer you the right way, just like we're all trying to do now.
I didn't intend to be this long winded, I just wanted to make sure you understand how a locker works and what to expect when you install one.
Good luck!