I had a similar problem with my '94 ADD system. throw the truck into 4x4 and the light would flash 4-6 times very fast, after randomly trying to engage 4x4 a few time with fingers crossed the light wouldn't turn on any more but i would still hear the click behind the dash, indicator switch was getting power . ended up having to go completely thru 4x4 vacuum system and that solved the problem. I apparently had a dangling vacuum line and when i had it serviced they plugged the line in the wrong nipple, not exactly sure how this happened but it solved the problem. If you have a 3rd party ADD switch on your front diff this can also lead to problems. i switched mine, it had a Dorn (purchased from auto zone i think) part number on it. the one from the parts store leaked badly and was not showing the proper resistance on the volt meter.

Just based on my experience with my ADD system i would trace all of your vacuum lines back to make sure they are not cracked or in the wrong place FSM vacuum diagram kinda is a drag but it will help you get the job done!