Locking rear differential is all you will need from what you have said. If a rear wheel starts to spin (like on a boat ramp), the locker engages automatically and locks both back wheel together.
Open differentials are what most vehicles have, even 4x4. With open diffs, if one wheel loses traction (spins) then most of your power gets lost to that wheel. You get stuck. Locking diffs lock the wheels on the axle together, so if one wheel on that axle is spinning but the other has traction, you will continue moving forward.
You do have to be careful when cornering and applying throttle with an auto locker. If your outside wheel starts to spin the axle will lock and your rear end will slide sideways. Most people over 18 will not have a problem with it though, because you have to drive like an idiot to do it. There are thousands of lockers on the road and Chevy/GMC wouldn't offer it as a factory option if it wasn't safe. It looks cheaper than I thought too, $350?
http://ask.cars.com/2009/06/silverad...ferential.html
Good explanation here:
http://youtu.be/q-rQTHMVAuw