OK here's an update with some not so good news and some good news. We're driving east from Denver across Colorado and Kansas. Air temps just over 90 degrees. Coolant shut off valves made a slight difference. Maybe a degree or two cooler at the vents but still 60+ degrees at the vents at speed. Not cool. Stopped at a picnic site for lunch and let the truck idle. Vent temps in low 40's. Soon as we got back on the road, vent temps went up to mid-60's. So I was trying to imagine what might be causing the problems at speed if it wasn't hot coolant flowing thru the heater core. I speculated that maybe the condenser wasn't getting adequate air flow at speed. We stopped in St Francis for fuel and I looked at the condenser. Mine has a rubber flap that extends down in front of the condenser just behind the bumper. I suppose it's there to protect the condenser from road debris but I thought it might be disturbing the air flow at speed so I bent it up and tucked it behind the bumper and pulled out on the highway. Presto, problem solved. Vent temps now at 43-45 degrees at speed! In fact, it was freezing us out. Had to turn it down. I think what is happening is that the rubber flap bends backwards at speed and directs the air under the condensor so it doesn't work. At low speeds it hangs straighter and the fan can suck air up thru the condenser. Anyway, it looks like my problem is solved. I'm assuming that rubber flap is OE. If so, it's a stupid design. So if your AC is not cooling well at speed, get that rubber flap out of the way and see if it improves.