Difference in rotor temp could be caused by a sticky caliper, swollen brake line, dragging parking brake shoe, rusty parking brake cable, or worn bearing. You won't know until you get into it. I like to replace rotors in pairs, but as long as they have good surfaces and are within the thickness spec, it won't matter much if you want to replace just the one. If a caliper is dragging, and they're both of an age, then I'd replace both. But you can usually do fine replacing just one.