I use this site a lot to identify problems with my avalanche and I'm very greatfull to all the help I've recieved. I've been trying to figure out this service 4wd problem for a while now and I've finally got it. I have not seen this solution posted anywhere so I have to put it up. Most people were telling me to replace the 4wd selector switch, but I didn't want to do that without knowing if it would fix my problem. I took it apart instead and found loose solder connections on the plug pins the connect to the board. If you have a soldering iron this was a very simple fix. thanks again for all the information.