Since they were recently done, just wipe them down with a damp cloth, and then repaint. Unless there is oil based paint on the walls, and that seems doubtful, since its a recent paint job, you do not need to primer, unless you really want to.

Thank you! I thought I may need a primer since I am going from darker (medium gray) to light (off-white), and due to the somewhat glossy nature of our current paint job (satin enamel from Behr, which is fairly glossy).

Going from dark to lighter, a coat of primer might be a good idea. It will help your off white paint to cover easier.

Prime the walls with white Zinsser Bulls Eye 1-2-3. (I've used Zinsser, Behr Multi-Surface, and Sherwin-Williams Multi-Purpose and I liked Zinsser the best.)