Ive got a 02 blazer zr2 and ive had problems with the 4wd working since ive owned it and am trying to fix it now. It has the 4 button and when i press the 4hi or auto4 the light just blinks a while and then goes back to 2hi, but when i hit 4lo the transfer case engages but the front diff doesnt engage. Im lookin to change the 4wd actuator under the battery plate to see if that lets the 4lo lock, but not sure why the 4hi or auto4 wont go in. Lookin to get some insight if it might be the tccm, encoder motor or an actuator. Also a few weeks ago i tried the 4lo and it shifted but didnt lock and the lights went off and the service 4wd came on, and it was stuck in 4lo. I unhooked the tccm and pluged it back up and the lights came back on and was able to put it back in 2wd. Not sure what the problem was there.

 

Check your actuator vacuum solenoid on the fire wall. See if there is vacuum going in, then see if the switch opens when you hit the 4hi or 4auto button . You can also just disconnect both hoses, if you have vacuum on one side, put hoses together and see if your front wheel locks. Then you'll know its the solenoid