I have a 2001 Blazer, One day I was coming off the highway and there was a "siren" noise coming from the front end of the truck. I determined it to be the differential lock. In 2wd it makes the "siren" sound while driving in four wheel drive it does not. But when I use 4wd to climb my driveway with snow and ice the front differential engages and disengages repeatedly and it doesn't fully lock in 4wd. I removed the battery tray and had someone switch it back and fourth from 2wd to 4wd. It pulled in and has the same constant vacuum. I don't think there's a fault with the lock actuator. What do you guys think the problem is? Any help with be appreciated! Thanks!!

 

The problem is low, or inconsistent vacuum to the axle actuator under the battery tray. 3 or 4 button switch on the dash for 4WD?

 

I have the 3 button switch. 2wd, 4high, 4low

 

There is a tech article with this exact scenario explained in the title! Check it out!