I had what I considered to be low grades during my earlier undergraduate career, generally C's and D's, including C-'s. I never went to grad school, but I took more advanced undergraduate classes later. I did retake some classes. Anything with a D. I only had a C+ in organic, but I chose to retake the course. I generally am not a person to advice people to retake courses (with the exception of prerequisits with unacceptable grades [<C-]) , especially if it is only one or two classes with lower than B-, but I felt that organic was definitely viewed differently from other coursework by medical schools. It was three years from the time that I took organic I to time I did the retake, and I received an A with a different, harder instructor. So I did notice an improvement in my grade, but I think it was a matter of maturity rather than repetitiveness (although with certain aspects of the course it did help to have heard the nomenclature used before, etc.).
Because you're pursuing a graduate degree now, and because you're doing very well, I'm not sure if you should retake organic. It depends on what your program is (i.e a medical sociology program vs. a biomedical sciences, biology, or science-based MPH) so that admissions comittees can have comparable scientific coursework to evaluate. If it is a non-science program, then retake. The other strategy I used when determining to retake a class, was to find out if there was an advanced science course which covered the material, and that mastery of that class would demonstrate an understanding of and proficiency (i.e. instead of retaking bio II which I received a C in, I took molecular, cellular biology, or vertebrate physiology). In your case you may consider taking biochemistry (and maybe biochem II) which are direct applications of chem II and organic chemistry. Also, I tried to get a high score on the MCAT in the area I had the most trouble with gradewise (physical sciences), which backed up the retaking of other classes. But if you've been taking graduate science coursework, I would consider just focusing on a solid MCAT score in both areas, and using your grad work as academic credentials. If not, retake organic I and II, and consider taking at least biochem I. I hope this helps!
-Aaron
