Service 4 wheele drive !!!

I have a 1999 chevy z71 4wheel drive, 119,000 miles on it. It has a 5.3 v8 in it.

My question is the service 4 wheel drive light comes on until you cut truck off. Then it might be wks. before it comes on again. Any ideas?

When it is on you can not change to 4 wheel drive.

I have notice before the truck sounded like it was in 4 wheel but the push button said it was in 2 wheel.

thanks,
Barry