Originally Posted by
MASTERNC
I'm fairly sure all of the terminals are now connected inside security (unless construction is affecting that). Might it work to take the American shuttle from A to F and then walk to D?
Yes inside security is no problem, you can walk airside from A to F, I do it all the time during longer layovers to get some steps in.
But in the situation above the OP was (potentially) arriving into A with checked luggage, then connecting onto a separate ticket on UA out of D. They would have to go landside from A to D with their checked bags to check-in again and drop their bags with UA. This is not possible to do all inside.