congrats man! i had a 1st year nissan when they changed the name from datsun.. damn reliable trucks.
the rear window being open with the front windows down allows for less drag because the air has an escape route.
on the rear window, think of it like a convertible top being open at the top edge of the windshield, or the aerodynamics of the back end of a car where the spoiler redirects wind flow.
the air on a pickup goes over the roof and drops where the back end is a straight flat drop off. where you have that back window, you also have the bed of the truck where the air catches and swirls creating something of a vortex that generates drag. It's difficult for me to explain because I visualize it. It'd be easier to draw and show.
Maybe you can google it and find a diagram somewhere showing airflow over the top of a pickup to explain drag at that back spot.
this is why a lot of people use a taneau cover or take off the back gate to reduce drag, and use a small spoiler on the truck roof to redirect flow.
there are actually several theories on this and all of them speed related. at speed the flow of air in the bed actually works its way to the front of the bed and gives the truck a slight push increasing mileage with the tailgate up. of course daily driven the average is probably better with it down. but then you look like a dork...