2008 silverado 1500 four wheel drive problem
 

When i switch my truck into 4hi it takes a little longer for the light to stop flashing then i noticed my front tires are not pulling and shortly after that the service four wheel drive light come on. Any ideas on what might cause this thanks in advance. It does the same thing in 4low the truck gears down but the front tires don't pull.

How do you know they dont pull? When you have it in 4wd (on concrete) and you turn the wheels can you feel the truck start to grab and make a moaning sound?

No I was in mud trying to back a load up a hill but the front tires would not pull.

Like the other poster said, I'd put it on a hard surface (concrete) parking lot. Put it in 4wd and try turning hard left then right. You should feel and hear the front tires grabbing and at the same time the steering wheel will try to turn back to center.