There is a part called an actuater. It looks like a plunger. They are natorious for dry rotting and the four wheel drive being engaged is on a vaccuum system that involves the actuater. Once that gets a hole in it, it stops the vaccuum and will not engage. It costs about 65 dollars and is about a 15 minute job to fix. It is located under your car battery. Remove your battery and the battery plate that your battery sits on and you will then be able to see the actuater. Once you see it, it is self explanitory on how to remove. Call your local gmc dealer to obtain part.
Broken rotor button.
do you engage 4WD manually or via the button on the dash? if its the latter then your actuator switch is probably bad. tweek
could be the 4 wheel drive module located behind the glove compartment. If you don"t hear a click when you press the button to engage the module isn't working.
the button for the hazards has a spring on a plastic piece that holds it on the spring may have fallen or broken that would cause it to stay on
the transfer case motor has to be replaced
To engage the 4x4 manually (VTM-4), you would need to be in 1st, 2nd, or reverse gear. In those gears and while stopped, you can then press the VTM-4 button, and will engage 4x4. It will only be engaged when around 18mph or lower. Above that, and it will disengage. It will re-engage when it dips down to that speed again. To turn it off, you can just press the button, or shift to a higher gear Keep in mind that even with the button off, VTM-4 will engage occasionally when there's slippage.
It may have the original transfer case that was recalled
i had this same prob u have a stock anti theift device ( alarm, it will cause it) i hit my alarm button a few times charged the battery and it is good to go now. not sure if that was the cause didnt really get in to it to much but u might wanna try that
Sensors got wet
faulty actuator switch. GM had some problems with these.
Starter selenoid no good
Is your transmission fluid full?