>>> What do you mean 4wd ranges?
Make sure it shift to / out of / drives OK in 2 Hi, 4 Hi, A4WD, 4Hi, and 4Lo. To get it into / out of 4Lo, you have to be in Park or Neutral. If the light on the selector switch flashes and then goes back to the previous gear range (and you hear no engagement, etc.), then that indicates a problem (and you can find many posts here about tracking down whether it's the switch, the encoder motor, or something else).
In 2Lo, you should not drive too fast (under 20 or so), and cornering will produce crow (wheel) hop on dry surfaces.
When you find yours, and if it has 4WD, the first fluid to replace, as soon as you get it, will be the transfer case. Use the GM Auto-trak II (blue); it's a little more expensive (if you can only source it from the dealer), but it gives the best result over others, as evidenced by others on this board. You'll need (2) quart bottles. If you DIY, make sure you can loosen the fill bolt first (you can check the level before draining it this way, too - to see where it's at, if you might have to look for a leak, etc.).