When I tried to shift my 98 Expy into AWD/4WD today nothing happened. The light on the dash came on and wasnt blinking. I heard the transfer case shift in but Im still stuck in 2WD. After all the snow we got in the northeast, I need to get this fixed asap.
Anyone got any ideas?

 

How about the front dfiferential shift not working. Not sure if I have it right, but prior to 99 there was a front diff disconnect, then in 99 Ford did away with it. It sounds like your TF is shifting but the front diff is not.

 

You have it right. In both 97 and 98 there was a vacuum actuator on the front differential that operated a lever to engage and disengage the axle from the ring gear inside of the differential. If this actuator fails to engage the ring gear to the axle, the front drive shaft will turn, but the power does not get to the wheels. There is a vacuum line that runs under the battery for this actuator. The acid and corrosion from the battery is known to eat through this vacuum line rendering the actuator inoperable.

 

Ok, it looks like the problem is the vacuum solenoid on the firewall. After pulling the blue vacuum line and applying a vacuum pump to it the front axle engaged and it held 25in of vacuum and showed no signs of leaks. When I measured the vacuum coming out of the solenoid I barely got 8in and I got 20in on the 2wd (pink ) side so I guess its leaking.
Thank you for pointing me in the right direction.