Hi -- First of all, congratulations on your service record! You sound like exactly the kind of applicant top MBA programs are looking for.
Now, should you retake the GMAT? Ideally, we'd want to see you at or above the median for the schools you're applying to. Looks like Darden might be the "stretch" on your list: with an average GMAT of 706, you're substantially below that average now. If you think you could get your GMAT closer to 700 I'd definitely suggest retaking it, just to remove that concern.
Other schools on your list, such as Georgetown and Vanderbilt, have a broader GMAT range, and you are wise to add these great schools to your list as well.
The goal here is to be able to use your application materials to show your strengths to their best advantage, rather than having to use space explaining why your GMAT isn't an accurate indicator of your ability to thrive in a top MBA program. You have such a wonderful service record, and no doubt will be able to show lots of the leadership top business schools are looking for. Good luck!
Best wishes,
Margaret Strother
Stacy Blackman Consulting