I have a 2000 ZR2 Blazer with the 4 button 4WD. Before I discovered the common symptoms cause by vacuum leaks I could only get 4WD on the indication lamps (verified by performance) while the vehicle was cold. Once it warmed up I'd lose the 4HI/4LO indication lamp on the buttons (the light would go out or flash) and the the front axle would un-lock. After replacing all the vacuum lines I could find under the hood I have 4WD on command. One of the symptoms remaining is; although the 4WD is engaged, the 4HI/4LO lamp will intermittently extinguish. None of the buttons are illuminated.

Another issue I'm having has me scratching my head. When 2WD is selected the 2WD lamp is always illuminated. Occasionally when I encounter a slick spot; be it gravel or snow, and the rear axle slips, I hear a non-familiar clunk and feel the front axle or transfer case engage. After this engagement I have a hard pull to the right and when I make a sharp turn the truck feels like it's spooled. I have to select 4WD to get the symptoms resolved. I've tried to search this symptom and have found zilch.

Anyone have a clue?

 

This may not be related but I had a similar issue where at times I could not engage 4wd and sometimes when I was going down the highway the indicator light would flicker in between 2wd and 4wd hi. Sometimes no lights would be illuminated. Found out that when I replaced the encoder motor i had not tightened the bolt down hard enough that connects the wiring harness to the encoder motor.