Service light comes on when using 4 weel drive

it does not have 4 wheel drive it is only a rear wheel drive vech

It's a 1500 z71 crew cab 4x4

no it is Silverado 1500 small v8 2 wheel drive, no 4 wheel, no power anything, all manual, my little Mexican truck the delco rebuilt alternator, did the trick all is well again thanks for all the help