Chevy & GMC Vans

The Service AWD light comes on in my 2000 Astro, at weird times. For days I can drive it, and then when I'm doing errands, stopping and starting the van, it will come on. The next time I shut it off, and re start it, the light is off.
There is a relay or something, behind the dash, that I can hear click. When it clicks once, the light won't come on, but when it clicks 3 times, the light comes on.
I've read about checking the connections for dirt or corrosion, and I've done that. It still happens. I'm not sure if the AWD works, when the light comes on, but winter is coming, and if I have to replace the shift motor in the transfer case, I'd like to do it before it snows.
The cost is making me hesitant about just going out and replacing it. I wil, if I'm sure that is the culprit, instead of maybe it being the relay, or whatever is making the clicking behind the dash.
Any ideas?

Though this might not be your problem, my old transfer case had a slight leak and when the fluid got low the light would come on. Check your fluid level.
FYI, last winter I had the encoder motor in my transfer case replaced by Aamco in Salt Lake City at a cost of $1700. Turns out there was other things wrong that they didn't diagnose. After returning to Oregon (less than a month later) I ended up replacing it with a used transfer case. Used transfer case and labor from my mechanic was only $600. Point is, that unless you are capable of tearing it apart and making the repairs yourself, it's probably cheaper to buy a used transfer case than have someone repair yours.

Good luck.

When I took possession this summer, I did change the transfer case oil, with the blue stuff. The old oil wasn't bad, but I wanted to change it. the transfer case was full, and there are no leaks.
As for my mechanical ability, I'm sure I can handle almost anything, except a transmission overhaul.

When the light is on, you need to scan for codes. A basic parts store code reader won't see the correct codes. You really need to get a dealership level scanner. For the cost of doing that, I would be looking at just buying a new shift motor. If all else fails you will always have a spare for when/if it does fail.