I have a 1999 Chevy Sliverado 4WD pickup with 75k miles on it. On Sunday I was driving around on vacation in Nashville, TN and the "Service 4WD" light came on. I was not in 4WD and didn't notice anything unusual while driving it. I drove about another mile and shut it off. I looked for oil leaks and didn't find any. I restarted it and the light did not come on, but after driving about 2 more miles it came back on. I drove a couple more miles and shut it off. I restarted it and the light did not come on and has not come on again since after driving about another 300 miles. Any ideas about what's going on? The manual just says it means the 4WD may need service. I had it in 4WD about 10 days ago to go across a muddy field and it worked perfect then.