Our psych clerkship director, who advises us through the residency process and also is involved with residency selection, told us us that roughly a third of applicants that interview have failed a clerkship. Mind you, my home program is by no means a stellar program, but it's certainly a quality program at a big-name institution that will get you good training.
I don't know my ranking as it's not directly disclosed and I don't care enough to try and find it out, but I'm almost certainly in the bottom quartile of my class, got a mid 230s step 1, and have received more interviews than I know what to do with, including a few at very solid institutions. I did not "win" medical school. However, this all comes from the perspective of someone who has no interest in academia and is staying away from most "desirable" areas due to personal interest (e.g., NYC, Boston, LA, etc.).
A good buddy of mine scored in the 210s on step 1 and is probably overall an even weaker applicant than me and we've been largely interviewing at the same places.
I don't know about PM&R, but it's not difficult at all to get a quality psych position. Getting one of "the best" positions is another thing, but even then it's nothing like getting one of "the best" positions in derm, the surgical subspecialties, etc. etc..