What does service 4wd mean?

I recently purchased a nice little truck with the z71 package. I have had it for less than 1000 miles and it came on to a srvice 4wd message on the dash. I also noticed sometimes when i start the truck it will be on 4lo for no reason. it switches right back like nothing. I have taken it off road and it works just dandy. its not new, but its an 03' with 81000 which is not bad. Any ideas of what it could be?