1999 Chevy Blazer to Four Wheel Drive?

In may not engage into 4hi until you start moving.

Buy a Toyota. Seems everyone is having problems with their 4WD systems in those newer Chevy's. That's what happens when Chevy tries to build something they screw it up.

Anyhow I'm guessing that it hasn't been in 4WD for a while, so its going to be a pain to engage it. Try moving slowly forward and pressing the button sometimes the system won't engage due to the gear train in the transfer case not being properly aligned. Your vehicle should have shift-on the fly 4WD anyhow which you are "supposed" to be able to shift inot 4WD hi at speeds up to 45MPH, I don't recommend that and if you've done that in the past there lies your problem.

The 4wd HI engages when you start to move. The nice thing about these trucks is that you can switch from 4wd HI to 2wd on the fly. If you want to get into 4wd LO, you need to put the truck in neutral, push the LO button, then put it in drive to get there. Same thing to get out, put it in neutral and push 2wd or 4wd HI, then go back to drive.

First thing I'd check is the serpentine belt.

Try in Neutral...if that doesn't do it, try it in drive at a complete stop.