2004 Chevy Silverado 4 wheel drive service message

I had bought this truck used, certified, and have drove it for about 4 years now. Just this morning as I was going out to my truck to drive to work, i started it and the service center message read "4wd service needed". I have read the manual cover to cover, don't remember seeing what this could mean. My 4wd is a push button. This happened to me once before a few years ago as I was leaving for a trip to Minnesota. My question is what do you think it could be and how do I remedy it? Last time it came up after a few miles it went away and never came back. I rarely use my 4wd but when I do I need it. I noticed the last few times I used it, it kicked in a little rough- I could hear it switch over to 4wd. I have never heard it do that before. In case it is the transfer case, I didn't drive it to work this morning.