With quadrasteer, I am unsure the rear gears can be changed....don't have a bunch of feedback on those things though to support that claim yet......
I have to say, I was once a big block fan, but the price for the Duramax more than pays for itself over time.
I am also unsure what 2500 has Quadrasteer.
My vote would be go with 4x4 and still get the locking rear posi.
On Brett's idea, you would have to find someone to change the PCM programming on your truck. Although I do know several folks that can mod out the PCM on a gear change, I do not know of any that have added such an overdrive unit to the and modified the PCM (PCM is the brain child of the vehicle, it controls engine, trans, and monitors dozens of vehicle functions). I also have no idea how this could effect the quadrasteer. The reason I say this is that when I went from 2.93s to 3.73s I could tell that the car, besides the ususal things, acted much differently and I have an overdrive unit (speaking of that, doesn't your truck already have 4 speed). Anyway, lots of things to consider before cracking open that truck. Also, and this is just a thought, if you change the rear gears and have 4x4, don't you also have to change the front gears too?
At any rate, you can contact Bryan Herter, he is the programmer I use for all car mods that I do. His website is www.pcmforless.com
Bryan is a real gem!