Someone mentioned the exhaust & whether or not the truck has the catalytic converter still on or not? Make sure the exhaust is flowing freely - clogged or really restrictive will make the engine temp go way up if it's enough. It would make sense on the highway that higher prolonged RPM would need more exhaust flow.

Also, I used to have a '78 Dodge Super Coupe that always ran hot. Pulled the radiator & had it boiled & pressure tested & it was fine, reinstalled & same hot running condition. Changed coolant & flushed the system & still ran hot. One day, we decided to pull the engine & clean it up, repaint everything (car was for sale & we were bored), & replace all the freeze plugs while the engine was out. When we popped the first freeze plug out, that was our clue. Basically, MUD came out. We pulled them all & really hosed out the passages & a LOT of sediment came out! Installed new freeze plugs, painted everything, put it all back together & it never ran hot again. The crap inside the coolant passages was the culprit. Good luck.