There's been problems like this for decades with wagons. It's not only the proximity of the exhaust. With all the windows up but with the back open, as the vehicle moves the outside air turbulence creates a vacuum which sucks the exhaust with its carbon monoxide right inside.
If you run with the back open, all the other windows must be down all the way, and sunroof open if you have one. You need to create enough turbulence inside the truck to eject the exhaust air, which means enough to blow sheets of paper around- and maybe out of the truck. Not very comfortable in most weather conditions.
With the above in mind, you're probably OK if, say, you have to pick up something oversized at Lowe's. But I'd not make it a habit.
