It's possible, but applying/interviewing for residency programs during intern year is really tough. And a pre-lim isn't that helpful to explore new specialties unless you are set on an IM sub-specialty (pre-lim IM) or surgery (pre-lim surg, but you likely won't get any real exposure to surgical specialties--you just manage floor patients and do any grunt work needed). If you do a TY, then you can spend time in radiology, peds, neurosurgery, ID, PM&R, derm, psych...
Still, you need to ask yourself if another year will help you figure out what you want to do--what rotations will you do in a TY that you didn't do in medical school? If it's just repeating the same rotations, that may not be as helpful. If you want to take specific electives, you'll need to make sure the program offers them and that you can schedule it before next year's application cycle. (And you need to match into a flexible TY that actually has good clinical rotations--it's hard to match into a TY, and some of those are just really laid back years whereas some actually have good learning).
Still, it's ideal to apply to residency programs while you're in medical school. The LOA for "research" the above poster mentioned might be the next best choice, though I suppose if you did match to a good/flexible TY that might be better (it's usually not as hard to take time off in a TY vs a prelim, since TY's have more electives which allow vacation time). If you go that route, you'll need to apply to a lot of TYs, particularly applying to TYs that are also less competitive (generally the ones not in or near big cities). TY's in big cities tend to be notoriously competitive (unless they're really rigorous programs). Still, there's not a whole lot of feedback on SDN on good TY programs (mostly just which ones are really light years), so there's still an element of a crap-shoot on whether you'll get the exposure you want. A TY could offer the elective you want, but who knows if the elective is any good? What if it's just pure shadowing? You won't accomplish anything in that month you couldn't accomplish right now.