what could cause service awd light on 2000 chev astro to come on ?

Aside from the all wheel drive needing serviced there are tons of possibilities. Instead of throwing expensive parts at it take it to a shop with the proper computer to diagnose it. I'm not kidding on the expensive parts thing.

Any scan tool can read the codes the computer has stored in it to turn on the light

Not necessarily these are stored in trans codes and most vehicles have different computers dedicated for the engine, transmission/transfer case, abs/ brakes, and one for the rest of the wiring like headlights horn etc. The standard actron cp9145 is the one I have and it will only do check engine codes. The other 3 have a separate scanner. The only scanners that do it all are $1000+ like the snap-on modis. Not all shops have those I had to take my step moms jimmy to a shop BC the 4wd motor went out and my scanner brought up no codes but the all the 2wd/ 4wd buttons were flashing. I would almost guarantee he is asking bc auto zones scanner wont read it.

I just looked it up. I thought the AWD didnt have a TCCM like the 4x4's. That was my mistake. A GM dealer or someone with a very fancy scan tool can check it.

How do I service my all wheel drive