service 4wd light

i have a 2000 silverado x-cab with 82000 miles.  every couple
hundred miles or so the service 4wd light comes on the dash board
display.  i dont know if its just a bad sensor or what it could
mean.  i dont have an owners manual so i dont know if there is any
kind of matienence that i need to do.  i would appreciate any
input.  thanks.

Your switch is bad. Common problem. $40 part.