It was 110°f (43c) where I live yesterday and I have a very similar situation.
When I purchased the expedition I was expecting the AC performance to blow my 20 year old Honda out of the water. . . . Shocker, it does not. . .
Looking at the 2 vehicles there are several differences that I would bet make a rather big difference in performance.
First my Honda has 2 electric fans that kick on high as soon as you turn on the AC. The interior of the Honda is MUCH smaller with a lot less ducting to get the air where it needs to go. It has no rear AC.
The Expedition has a electronically operated viscous fan clutch, it will only ever spin as fast as the motor turns it, and my expedition idles close to 500rpm in gear. The PCM will only ever command 100% lockup under certain conditions, those conditions are rather extreme in my opinion and if you do manage to get there you might have other issues. Coolant temp: >260°F, AC high pressure: >240psi, trans temp: >300°F, all other conditions will command less than 100% fan speed. So removing the heat from the condenser might not be as efficient as an electric fan setup. But once you start moving and get a little RPM they generally work better. I'm sure mine needs to be replaced and I might add aux fans to help cool the AC in the summer months, but we only get to around 117°f (47c) a few weeks a year out where we live.
The explorer your friend has is equipped with electric fans, with both vehicles idling AC on how loud is the explorer compared to the expedition fan noise wise. I never really did like mechanical fans because at a dead idle factory electric fans generally have superior cooling ability. However as soon as you get a little RPM into the motor the mechanical fan will outperform an electric every time. If you cant get the temps to drop any lower while traveling at a consistent pace then you might have other issues.
If there is no change with a constant drive there are several issues that could be the cause. There are various barriers and shrouds to direct the most amount of air across the condenser and radiator, if those have been removed or are missing your cooling performance could suffer.
Other issues that might have already bee mentioned.
*Weak AC Compressor
*Blend Doors
*Faulty Thermostatic Expansion valve (TXV)
*Low refrigerant
*Clogged Condenser
*Poor airflow across the condenser
*failing Cooling fan not drawing enough air across the condenser.
The shop who looked at your truck should be able to tell if you were low on refrigerant and if the TXV or compressor are good from the pressure differential.
I wonder what other people are getting for vent temps in similar climates.