I agree with all of the above posts. Like you, I had below average USMLE StepI, average step II, and below average grades my first two years. I did do a lot better my third year and did two away PM&R electives and an acting internship in PM&R at my school my fourth year. I also got honors in my Neurology acting internship (a related field). Other related rotations would include ortho, rheum, sports medicine, and maybe cards. (there's a thread about this topic)
I started planning for fourth year around February of third year. I would request certain attendings at your PM&R electives if you know of any "name" in the field to secure good LORs. I made it so my first PM&R away elective would give me a broad exposure to the field. (did P&O, TBI, Ortho, Stroke, Wheelchair clinic, EMGs, Pain, and accupuncture) then, I used the second rotation to go a little more in depth - did two weeks of TBI and two weeks of SCI. I would say two weeks is the minimum to spend with one attending to get a LOR. Of course, a month would be better. (if you have an interest in one aspect of the field of PM&R, by all means, do a rotation at an institution known for that subspecialty. If you already know which programs you like, do an away elective at a "reach" program.)
i also sought out the one physiatrist on staff at my school and started working on a small research project. (case report) I had also gotten two publications (just abstracts and posters) in the field of pain managment through my Anesthesiology department.
I also spent a pretty good time and effort on my personal statement - which many PDs and interviewers commented on as one of the best they had read. (just be honest and concise.)
So the bottom line - it is not too late to start working on ways to fortify your application. I really would not take a year off unless you really want to do an OMM fellowship. If it is only to make your application stronger, there are other ways.
-work on your personal statement now - think of the things that make you different and emphasize your committment and dedication to the field of PM&R. It also would help to get a few physiatrists to read your statement.
-find a research project. it can be really small but just having that on your CV helps. - publication would be a plus but it's probably too late for that.
-look for mentors and letter writers. I got three PM&R letters and one medicine letter. PM&R is a small field and the PDs will most likely know the letter writer or the institution you rotated through.
-do well on those away rotations and get good evals - make copies of those evals if your school doesn't include them in the dean's letter. Take them with you to interviews - just in case.
-I don't know much about the COMLEX but I took Step II early (july of my fourth year) to make up for my mediocre step I score.
-get your application in early - mine was about a month or two late - and I think I missed out on a couple interviews because of that. That is my one regret.
-make contacts. the fact that you are on this forum asking for advice is a great start. Catch a PM&R resident, buy him/her coffee and pick their brains. It's worth the few bucks.
I also went to the AAPMR conference and talked to a lot of residents. It is a great way of getting a "feel" of the field but if it is too expensive, it may be better to save your $$ for interviews.
-go on more interviews than you think you should. I went on 12 and ranked 10 programs. I think 6-10 is a good number. Some of the interviews were a waste of my time. If you do electives and you get the impression that they like you and you really want to go there, you may even be able to cut the number down to 5.
If you are sincere about your interest in PM&R (i.e. it's not your "back-up" plan) and you are energetic and passionate, I think it will more than make up for your grades. Most PDs did not ask me about my grades or board scores. They talked about my personal statement, my LORs, my activities, my electives in PM&R, and my research. And try to be optimistic and positive - that will take you far. Good luck!
hope this didn't come off like a lecture. Just passing on what I learned from my own experience. Like they say, "if it doesn't apply, let it fly!!". take it with a grain of salt.
p.s. - hi Whirley!! Nice to see you on the forum!