4WD Service light

Hi...I have got a GMC pickup 1999 Z71 with 5.3 V engine done 25,000 ..From time to time i got a 4WD Service light any body can tell me what dose it mean .. everythings seems to be normal however i have a differantial oil added recently!

I have a '99 5.3L Silverado and the same happened to me. I took it to Mr. Goodwrench. There was no problem with the 4WD. It took them 2 hours to determine that it was a bad switch. Cost: $200.

If you know someone who can check this out for a lot cheaper cost per hour, I'd try him. Good luck!