Service 4 wheel drive message

On my 2004 Chevy Silverado, the "service 4 wheel drive" message has come on twice. The first time the 4wd worked fine, but the dash light showed the transfer case in neutral while driving down the road. I checked all fluids and electrical connections all were fine, and the message went away. A couple months later the message appeared again, this time the 4wd would not work,
transfer case would not switch to 4wd. After a couple attempts, backing up and shutting the truck off and restarting it worked.( I really needed the 4wd)
The message has not appeared again. Any ideas?

Welcome to AF

Need to get it on a "high end" scan tool that can access the 4WD module. Post back both the "active" and "history" codes from the 4WD module. The DTC's are really the only way to diagnose the newer vehicles.