All great advice.
Sounds like the transfer case is stuck in neutral. If it's in neutral, the truck won't move, even with the transmission is in Drive. Hop on GM's website, in the Owner's Section, you should be able to download the Owner's Manual for free. Look into the correct operation to make sure you're not breaking anything (sometimes they are listed on the top side of the drivers sun visor). My first 4wd without manually locking hubs was an eye opener.
Basic Checks.
Check the fluid, there should be two plugs on the back of the transfer case. It should be level with the bottom of the top plug. If it's not, it should be topped off, but also try to find the source of the leak...
If you're able, check the function of the transfer case shifter with the engine off, it should engage firmly into 2hi (like a manual transmission's shifter), if it does, it's not the linkage. If it doesn't, the linkage could be bent or need adjusted. Have someone move the linkage while you look at it underneath. If it's not shifting into 2hi with the engine off, you may be able to disconnect the lever and shift it completely by hand underneath. If it's still not working, it's internal.
Keep in mind that there should be a vacuum switch on the side of the transfer case that sends a vacuum signal to the front differential. If this is bad, it will keep it from working properly.
Good news though, transfer cases are pretty easy to rebuild with a bearing puller, possible hydraulic press, dial indicator, and feeler gauges. Instructions and a fearless attitude help.
If you're not confident with that, there's the option of replacing it with a used unit, probably less than $200 if you're willing to wait a bit to find a deal.
I'd go through the basics first... Make sure there isn't operator error, check the fluid, then linkage function.