You are on the right track as far as lean. As a side note, I repair/replace horiba 3 gas analyzers here in ohio. Probably what they are using to test your truck. Hi Nox is caused by hi combustion chamber temps. the egr "Cools" the temps down some. Did you happen to get a CO2 reading on your printout? CO2 is the a/f ratio, That's why there is no p/f for it. There for diagnostics only. Should read 14.7 if everything is right. Should pass also. Most new cars are hanging out around 15.5. I would be curious to see how close the CO2 readings are to your a/f gauge. At 17 that thing should be rattling bad. I would suggest richening it up some. Idle mixture screws can affect it at the low speeds it's being tested at. What will most likely happen is You'll richen it to pass Nox and then HC & CO will go up and or fail. It has a see-saw effect between the 2. When cars fail for high HC, Nox is very low. I would try going up a jet size, Take some APT out & see what happens. But at $20 a pop you want to be sure. Any shops in your area have a 3-gas? Also 50 degrees total with the vacuum seems a bit high. That can aggravate it as well. Did that truck happen to come with 2 smog pumps?
Good luck