2004 Chevrolet Trailblazer Question: Switching from 2HI TO AWD TO 4HI

When I try and switch from 2hi to awd or 4hi the light flashs on and off then throughs the awd light on in the dash??If I shut the truck off at night and then switch it into awd. When I start it in the morning it will go into awd and work fine.Hopefully someone can help me with this problem so it doesn't cost me alot of money....