2001 GMC safari AWD help

[Thealien]

I have a 2001 GMC safari AWD with just over 100,000 miles and it has been a fairly reliable vehicle. Right around 100,000 miles the service AWD light came on once or twice. There is a service bulletin about changing the fluid. I figured at that mileage I would change all the fluids front/rear diffs, transmission, and transfer case. The light did come on once or twice since but it seemed to be working. Today in the snow I realise that I have rear wheel drive only. ANy suggestions? Could this be the shift motor? Is there a relay or something that controls it?

I really can't afford a huge repair bill at the moment.

Thanks
Mark