Hi, I have a 2000 4 door 4x4 blazer with 157,000 miles. The issue I’m having with it is the 4x4 doesn’t seem to be engaging. I’ve put a new vacuum switch on top of the transfer case, and I’ve checked the vaccum actuator underneath the battery tray and it seems to pull in like it should when 4HI is selected. I can hear the click when I hit 4HI and the yellow light stays illuminated on the button like it’s working as it should (I have 3 button 4x4). When all 4 wheels are off the ground on jack stands, when 4x4 is selected the front wheels don’t spin but the back do, I hear a sort of grinding noise when 4HI is selected like something is trying to engage but won’t. the odd thing is, is when it’s in 4HI with all 4 wheels off the ground, even without touching the brakes the front wheels lock up and won’t spin by hand which tells me something changes when 4x4 is selected. I’m kind of at a loss, I don’t have any vaccum leaks that I’m aware of as I checked some vaccum lines and fixed one. When it’s on the ground and I drive through something slippery, the front wheels don’t spin even when the back are which tells me it’s not engaged. What else could it be? Thank you!

 

This will explain the system in detail.
4x4 Front Differential Operation Explained & Grinding from Front End - S-10 Forum

 

Ive seen the vacuum line come off the ball canister in the drivers fender and the actuator would pull but not quite enough with this loss of vacuum.