i have a 99 blazer and i have noticed that the4x4 does not work. I can hear it click in to 4 wheel but i have no power to the front wheels.

 

on my 95 I had a simular prob. it would click in but no power to front wheels, On mine there is a rubber plunger under the Battery tray. check and see if it is in good condition. The rubber maybe dried out.

 

The rubber plunger he mentions is the 4wd actuator.

First things first(and cheapest) check all your vacuum lines and replace as needed. There are three lines coming from the a switch on the drivers side of the transfer case, one of them is a vent, one of them runs to underneath your battery where the 4wd actuator is found(another possible culprit) and the other line gets vacuum for the switch. Check them all for cuts, and check the 4wd actuator for cuts as well. I had the same problem a couple weeks ago and it was the line running to the actuator, replaced where the line was cut and it still wasn't working so I checked it out again and it was cut in a second place behind the distributor. I replaced another section and now it works perfectly.