When you exit the Customs Hall at MIA and follow the signs for "Connecting Flights" you'll be directed to a TSA Checkpoint for Terminal D that DOES NOT have Pre-check. Our experience has always been that if you ask one of the attendants or airport personnel where the nearest "Pre-check line is located they will plead ignorance or tell you it is not operating that day.

Don't believe them. You have two options: You can exit the terminal and proceeds to the upper level and walk (toward your left) to the TSA Checkpoint for Terminal E - it's right opposite the registration desk for the MIA Airport Hotel and only about a 3 minute walk from where you exit the the Customs Hall - it's a little confusing because they have the direct route closed off but you can get there quickly by going down one level and back up again. We've hardly ever encountered any long lines for Pre-Check at this checkpoint. Your other option is to go to Checkpoint 2 in Terminal D (just passed the ticketing counters near Gates 23 and 24 if I remember correctly). It's about a 10 minute walk (to your left) when exiting the Customs Hall. The TSA pre-check line there moves relatively quickly but can be crowded.