4X4 front axle not engaging?

The easiest way to work on these is to work backwards. Take a handheld vacuum pump and connect it to the actuator. Apply vacuum and then verify the axle is engaged - the 4wd light on the dash should light and if you shift into 4wd and then turn sharper on pavement. It should bind up and crow hop? Yes= axle is engaging, and transfer case is operational. That rules out the case and axle disconnect and the actuator and cable. Next - use the gauge on the pump to test for vacuum at the line on the actuator. With it in 4wd and the engine idling there should be a strong vacuum there. I'm thinking it will be weak or not there. The next stop would be the vacuum switch on top of the transfer case. It has 3 lines to it, one goes to the actuator, one goes to the engine vacuum source and the other is a vent. In 2WD the switch vents the actuator to atmosphere to unlock the front axle. In either 4wd mode it sends vacuum to the actuator. I've replaced a lot of them because they fail. However, you want to check the vacuum source line first. It runs from the intake down the rear of the engine and those lines don't age very well, they split or crack and now the 4WD doesn't work. So, check that line for vacuum, you can reach up and pull the 3-way vacuum connector off from under the truck. It's right above the spot the T-Case shift lever or motor is located. Generally, if I've got this issue, I just get the actuator as they fail so often that even if it isn't the current failure, it soon will be. GM part # 89059420. On that 3 port connector, if you have someone start the truck you should hear and be able to test the vacuum to the selector. Now there have been a few 98 Blazers built that had the later system adapted to them. That version just puts a solenoid valve in the line between the actuator and the selector on the transfer case. It's easy to find because 99% of them are within about 10 inches of the actuator stuck to the inner fender or tied to the harness. Those can stick closed. The system works okay, but I had enough of issues with the ones I had that I opted to convert them to a manual cable that I could pull inside. The advantage of that was that I could have a low range function in 2WD. Not something for everyone but there are times it was handy.

Normally the vacuum lines come off the engine through a check valve and then there would be a T that feeds the 4WD system. The other side of the T then goes over to the fender where the vacuum tank is. In that line will be another T that feeds to the interior for the HVAC system. Leaks or disconnected lines can cause problems. If it were mine, I'd start at the engine and replace the lines, normally that vintage had the nylon lines with the rubber connectors, so someone has been there, likely because the connectors like to turn into goo and fail and regular rubber line is less money.

Yeah, that would be a charging problem. Glad to hear the 4WD works. Likely had to relearn because the vacuum was leaking before, and it adjusted for that.