It is at my school; I'm going to take it right before starting my post doc between my third and fourth years of med school. The bigger question is, have you finished your rotations yet? People normally take Step 2 after completing their third year. I don't think it's a good idea for anyone to take Step 2 without completing all rotations, but especially for you since you're counting so much on this score.
Studying for Step 2 twice sounds about as much fun to me as poking myself in the eyeballs with a red-hot steel chopstick. I think you'd be better off putting that energy and time into honoring your rotations, and then take Step 2 right after. If you do well in your rotations, you probably won't have to study as much for Step 2. Does your school have shelf exams or other tests that you have to take for each rotation? Those should give you an idea of where your knowledge base is at in each field.
