91 astro AWD No front drive

I just bought a 91 astro all wheel drive and noticed that the front wheels were not turning when I had it up on the hoist, Is there switch that engages the front differential? If so where is it? or is this engaged by the front wheel speed sensors? My manual dose not show much on the AWD model and no schematic at all.
All info will be greatly appreciated!!!!!!!!!:banghead: