'Service 4W Drive' light comes on occasionally

A dashboard light comes on sometimes that tells me to service the 4W drive. I’m not sure what that means. I have checked the fluid in the transfer case and it is clean and full.

Maybe there’s no connection, but my mileage has been really bad for the last few months and I was thinking maybe the front axle was partially engaged because I hear (or think I hear) a noise coming from the front end that sounds like the transfer case is engaged. I took it to a transmission shop and was told that everything seems to be working properly.

Any ideas about what is going on?