I haven't heard of a post-bacc program in the Caribbean? Just curious though, why the Caribbean? I would speculate that most formal post-bacc programs in the US would make you competative for Caribbean schools.
Actually I believe there is one at Ross but I do not know much about it. It something where you do your prereqs and is geared to get you into their program. Its been a few years since I've looked at those options though. I would do a search in this forum for caribbean and see what you come up with.
Don't do a post-bacc in the Caribbean. Any degree you get from there (other than the MD) will be virtually useless. There are plenty of post-bacc options here in the States, even if your grades are awful. I've heard of SMPs here in America that take people with GPAs below 2.7, it's not unheard of. Plus any program in the Caribbean is going to be expensive, and you won't be getting as valuable an education there. Not to mention the funding issues, you will have a much harder time financing an education outside of America. If you have any other choice, stay in the States and do something here.
Do a post-bacc here and get your MD from Ross or SGU if you want. I think you would actually be more competitive for a Caribb MD school by doing your post-bacc work at a school here, and you could probably save money by doing it here as well.