Talk to the sales guy again. He should be shot for not telling you that there is a conversion kit that allows for the new style caps to fit. Not 100% on what is different, but it will run ya about $50.
I have friend of mine that has a 2003 ram, and we ran into the same problem. I just wanted to replace the one missing cap on her truck.... but hell no, cant do that.... Just have to buy the kit and replace all four.