Grumman LLV

Jeep and Ford have had postal truck contracts in the past. I assume the defense contractors are simply bidding lower than commercial makers are willing to go?

Neither of the engines these came equipped with are diesel, they're gas engines. They also have some very specific features that make them well suited to mail delivery.

Saying that, they do have their problems, like no AC, poor heaters, and bad MPG. The new EV replacements are pretty well liked by USPS drivers, from what I've read.