2002 Dodge Durango Service 4x4 light

I just bought a 2002 Dodge Durango (really cheap) and it needs a little work done to it. The battery was unhook when I bought it, I had to put the positive terminal back on the battery before I could start it. I mentioning this because I know unhooking the battery can sometimes reset the computer and certain other electrical components. But shortly after leaving the used car lot, maybe 15 miles, the AWD light went off and the "Service 4x4" light came on. I tried to put it in four wheel drive just to see what would happen and of course, nothing did. Is this something that is common, serious, or what. Thanks.