I don't think you need a postbac, just a buffing up of the ECs. Two of those schools would tend to want to see more than summer research. One would probably like to see some rural type experiences and it's not too late to shadow a rural doc over the winter break and send in an update letter about it (I've seen this work for that school).
You've got some leadership, but possibly not much in the way of actual clinical experience (when 1.5 years is average on applications). You don't mention physician shadowing at all, have no teaching/TA/coaching/mentoring experience, and no mentioned nonmedical, noncampus community service. I'm not saying you need all of this, but as much as you can manage to set in motion will be helpful, possibly for this cycle, and certainly for the next if you need to reapply.