Hello. Here's my advice, for what it's worth:
1) Finish the JD. Use it, don't use it -- there's zero advantage to dropping out of the program now, and lots of obvious downsides.
2) Don't worry about your JD GPA. No one cares. You got into law school, a rigorous graduate program, you will have finished law school by the time you apply (see #1 above), and none of the classes are B/C/M/P, so it's irrelevant. My grades were mediocre, and my school used a weird scale, so by the time AMCAS finished translating my transcripts my GPA looked like a 2.7. But I'm still staring MS1 in two weeks, so there you go. Not one of my interviews asked a single question about my law school experience, grades, whatever.
3) I've got to disagree with Q -- I don't think you can start studying pre-reqs or start prepping for the MCAT yet, even as a 3L, unless you're a lot smarter than I was. But you can and should start buffing your "other stuff" -- shadowing docs, volunteer medical experience/work, and community service. Come to think of it, you should start volunteering (during school, if possible, and definitely over the summer(s)) somewhere you can get experience in some area that overlaps between medicine and the law. For example, years ago, before I ever considered going back to med school, I did a lot of work for the local AIDS clinic. It was all legal/administrative, but it put me in direct contact with ill people and their families. All of my interviews addressed this experience.
4) You don't have to be a lawyer per se after you get your JD, but I would do something for a couple of years while you prep for med school. Otherwise you run the risk of looking flaky: "I want to be a lawyer. No, a doctor. No, a chef. No, a lumberjack!" You can work for a little bit, make some money, get some experience in the real world as a professional with responsabilities who has to work with clients under stressful conditions. These are all good things. Then you can start to find ways to "shift" your focus from law to medicine, like by getting involved in county bar associations or professional societies that overlap: med mal issues (plaintiff or defense), bioethics, AIDS or elder law, etc.
Good luck and feel free to PM me if you have any questions. Remember, truth is beauty and beauty is truth; that is all you need to know. Well, that and res ipsa loquitor. And also dicta, it's all dicta -- 2S4MS.