OK, so our fine friends at Atlas will hook you up with a 4.3:1, 2 speed t-case for just a shade of 2300 dollars...not including installation, of course. I imagine if you were to get their 3.8:1 it would be about the same money.
If you want new differential gears, you're talking about a replacement for front and rear - you'll probably want to consider new gaskets for your diffs and, of course, you'll be swapping fluid as well. Bear that in mind, along with the cost of installation. As an example, if you buy the gears from the fine peeps at Yukon they'll run you about 600 bucks for the front and rear pinion sets. Jump to Motive gear and you can likely find a set for around 450-500 if you look around. There are others, of course. Just a rough idea.