Oh, yes, I'm aware of the "Postal Police," (officially the U.S. Postal Inspection Service) but I would expect them to roll around in, perhaps, a 2WD Postal Service Jeep, or maybe something like an Escape, or Explorer.
Gotcha; well, it kind of makes sense for an entity that still rides around in vehicles last produced in 1994 to use a Crown Vic for their "law enforcement" angle, no? Seems like it fits in perfectly.
Lol, all jokes aside, I've never seen a postal police Crown Vic in real life. A quick google image search shows that they do exist.. however in an outdated livery apparently. They even had Caprices at some point in the past! But like you say, it seems they mostly drive around in explorers.
One of my customers in the local USPS vehicle maintenance garage. They usually have two postal police cars there at any given time I show up, (not being repaired, they're just there. I don't know if they hang out there or what) and I've never seen a Crown Vic, jus Durangos and maybe I saw an explorer once, but my memories' a bit hazy.