I agree with Fletch ... you can trust the SkyTrain to get you from gate to gate in the "D" concourse in just a few minutes, IF you are outbound. Just don't make the rookie mistake of trying to save a little time by walking between gates "just because" the numbers are close together. This concourse is HUGE, and just walking 5 or 6 gates could mean a 1/4 mile or greater hike, so TAKE THE SKYTRAIN.
Coming back to the USA from the Caribbean, you are at the mercy of the gods of bureaucracy (TSA, Homeland Security, Customs, etc.). You will be going through immigration control, then reclaiming your bags, then through Customs, then re-checking your bags, then off to your gate. Expect a fair amount of yelling and condescension to be directed your way. I would plan on a minimum of 1 1/2 hours to get through this cattle call. Oh well, that's the price we pay I guess.