All I see on NJ postal vehicles is a number - no NJ state plate.
USPS owns delivery vehicles, 5 ton trucks, VTRs, seized vehicles, plain old passenger cars and other vehicles of all types ~ i've used a non federal state plate on a vehicle where it was important it not be identified as a government car ~ lest a postal employee be misidentified as a 'revenooer' and get shot at!
For all you guys out there who think Washington actually put down the Whiskey Rebellion, you are wrong ~ he simply started a war neither he nor any other President could ever win. it's still on
they also stencil numbers on vehicles ~ particularly if they don’t have plates. those plates last forever though so there’s no sense not using them if you’ve got them.