My father delivered mail for over 30 years then retired. He had a foot route the first 15 then we moved and he got a rural mail route the last 15. He tried most every kind of vehicle from step vans to suburbans, station wagons, blazers, even a used mail truck from an Gov't auction (the used mail truck was pretty beat but he got it cheap enough and replaced the motor and got a few more years out of it) He never had a pickup because not enough room for all the mail on his route.
His last several years he bought a used 4 door Checker Cab that had a rebuilt 6 Cyl Chevy motor. Other than brakes and regular oil changes that car lasted many years. So he bought a brand new Checker station wagon (there used to be a Checker Car dealer not to far from him) and kept the cab as his back up car but never really needed it. It was not 4WD but with rear chains on in the winter it never let him down. The Checker outlasted any of the previous cars he had which is probably why so many cab companies used them.
Checker driver trains were odd though, the cab and the new one he bought had the same drive trains which consisted of a Chevy 250 Cu In 6 cyl. mated to a Ford C-4 and a Buick or Olds rear. Body parts (fenders / doors etc.) were also interchangeable for over 20 years. Too bad they went out of business.
Rural delivery is very hard on any vehicle. Brakes and regular oil changes were the biggest problem my Dad had. He usually did a brake job and oil change once a month due to the miles and constant stop and go.