Any smog experts out there?

A buddy's '95 Dakota 3.9 5speed flunked it's latest smog test. Only 3800 miles since it's last smog test two years ago when it had a new catalytic converter installed. All the HC, CO, and Nox readings are very similar between the two tests, both at 15MPH and 25MPH, except for the 25MPH Nox reading. It went from the 200's to 837 with a 753 max allowed. Engine is running cool, Idles fine (no apparent vacuum leaks), EGR system is working properly, no codes have popped. I need to borrow a scan tool to test the 02 sensor though, as maybe it's on the verge of failing? Anything else you can think of that is a likely suspect for this vehicle? I'll probably just replace the 02 sensor anyway as I think it's the original, decarbonize the cylinders, and maybe put in some real short reach plugs temporarily to lower the compression ratio a bit to help lower the combustion chamber temp. What else am I forgetting?