93' Jimmy 4WD problems

My jimmy is an all wheel drive, and it has a button press to switch between 4 HI and 4 LOW.

When put and nuteral, i can switch between 4 hi and low. i hear a noise from below the truck which sounds like an actuator or something, so the electric motor in the transfer case seems to be working.

I took it offroading, and got stuck in a spot a 4wd vehicle shouldnt have.

I took it over to my neighbors house and put it up on the lift enough that the tires were off the ground. We put it in 4 hi and 4 low, and in both instances, only the back tires would spin.

So what would be stopping the front wheel drive from not working. My neighbor the mechanic thought the previous owner may have found a way to disengage the front wheel drive for better gas mileage. If this is the case, how would we re-engage it? What else could cause the front wheels not to be working.

If its a cheap fix, Ill do it, but I really am not gonna spend 150+ bucks to fix the front wheel drive because the vehicle will just be used for getting back and forth to school and whatnot, and i can go without it.
(keep in mind, my family owns an autoparts store, similar to napa, and my neighbor has been a mechanic for his entire life)

fixed the problem, broken vaccum line.

hey my names mike i got a 93 jimmy 4.3 4x4 same thing same problem how did u fix the vacum line where is it located and how did u find out thats what it was-MIKE