All- thanks for the feedback. Signage at MIA is lacking in places, but good advice to find an elevator to the third floor and use the moving walkways from there before going back to 2nd floor in Terminal J.

On the way back I did take the terminal E elevators back to the 2nd floor to find the checkpoint by the Taco Bell. As mentioned above, it was 'regular' screening only (although they do have a Pre-Check sign there implying they offer that at times) and there were about 30 people in line. I walked down to the Terminal D Pre-check screening area and used the FL check-in instead.

Total time each way was about 15 minutes at most.