luv my truck

I have a 99 Silverado 4x4 which I luv almost as much as my wife and kids, recently the "Service 4 Wheel Drive" message has popped up. I have checked the fluid and looked for loose or broken wire and have found nothing. There are no unusual noises and the transfer case kicks into gear when activated. Any ideas what to look for? I have been told I might have a bad sensor, but can not locate one on transfer case. How do you test a sensor to find out if it's bad? Thanks