4x4 problem

[Mike1980]

I have a 2002 gmc sierra 2500hd that I'm having problems with the 4x4. The front axle is engaging and the shift motor on the transfer case seems to work but the driveshaft doesnt turn when I spin the tires. When the truck rolls the front drive shaft turns which means the front axle engages. I've taken the transfer case motor off and had someone push the button and it seems to turn. Any ideas what I can do or what the problem might be? The service 4x4 light never comes on till i put it in auto 4x4 and the tires spin for a bit.