OK, i used the search, and didnt find anything quite like this... I have a 99 2dr Blazer w/ the auto 4wd feature. When i put it into 4hi/ 4lo it seems to go in; the light stays solid and you can hear it engage, BUT there is no front wheel spin at all, even on snow/ice. Now i put it on a lift and engaged the 4wd... What do you know, the front wheels spin. I had a person hold each front wheel, and they couldnt hold them back from spinning. Yet, when the truck is on the ground the 4wd does nothin. I had a very snowy driveway the other day, and putting it into 4wd, even 4 low, didnt help at all. Yes the driveway is a hill, and yes i got stuck. Just spinnin the back wheels and nothin else. Ive pretty much excluded the vacuum system as the problem considering it has no problem on the lift locking in. Anyone got an idea of what else i should look at? Im stumped.

 

I know that you do not suspect a vacuum problem, but a good way to confirm that is to check the vacuum pressure at the front axle actuator. Do you have access to a vacuum gauge? If so, just for kicks, check the vacuum pressure at the actuator. You should have close to the same vacuum as you would find by tapping off of the intake.

Did you engage and disengage the 4wd system a few times on the lift to make sure that you didn't hit it just right that one time and have it engage? A sticking front axle actuator cable can be random...

 

Ok what you are saying is check the vacuum pressure at the actuator under the battery? Because i thought it went to a cable to the front axle after the actuator. I do have access to a gauge although i am in ohio currently, and my dads shop is in PA. Next week ill be back in PA so ill definately check the pressure to make sure. I did try engaging and disengaging the system a few times on the lift, and yeah it went in all 3 or 4 times. Could it have something to do with the axle and wheels being free while its up in the air making it easier for it to lock in or something? The thing is, if that was the case, i would think that it would engage once you start moving, or shift into drive/reverse.