83 Shevy won't lock in

[lud]

Heres the problem. I have a 83 chevy, When i bought it the 4 wheel drive worked. Well being in Denver Colorado I went to get the truck out after all the snow that we keep getting and went to put in it in 4 wheel the light comes on the front drive shaft is moving but the front wheels aren't. It has an automatic 4 wheel drive. Shift on the fly. I looked under the truck and it looks like it shifts alright. Was wondering if you guys could help me out. My wife is getting tired of me driving her toyota 4 runner. Any body know anything about this.