On a 90 you have the vac. switch on the firewall as you said. 91 and above it's on the driver's side of the actuator on the dif. and it's electric. With the ignition on, don't start it, if you remove and pull air on the vac. line that's attached to the switch the 4wd light should activate. Someone else will have to sit in the driver's seat to let you know because it will cycle on and off as you release air pressure. If it doesn't then it's the switch itself, the wire going from the switch to the dash or the bulb. But, if you already checked the bulb and you're sure it's ok then you can rule it out. Just to be sure, I would temporarily swap in a bulb that you know works. Most likely the vac switch over the wire into the harness if it still doesn't work. Just make sure it's definately not the bulb.