Do you mean should you retake Calculus II generally (i.e .life science version) or do you mean should you retake the version that you received a D in (I am assuming that this is Calculus for non-Biology majors/engineering and physical science majors, correct?)? I think you should have a second Calculus II course, but a life science course sounds fine to me unless you are applying to a very tip top allopathic research school and research intensive track (e.g. Harvard HST, etc.). I'm not sure how D.O. grade replacement works, so it would be worth seeing whether the life science version could replace the non-life science version if you are applying to osteopathic medical schools.