There are two devices that control NOX. EGR and reducing cats. Oxydizing cats cut CO an HC while reducing cats cut NOX. Most cats today do both.
The EGR cuts NOX by reducing combustion chamber peak temperature. So make sure that the EGR is fully functional. Apply vacuum directly to the vacuum nipple on the EGR valve with the engine at idle. The engine should degrade in idle quality, and probably stall. If it doesn't, remove the throttle body and clean the EGR passages behind it (good idea anyway, this is a common problem on these engines). Also make sure that the valve itself isn't plugged or the diaphragm ripped.
Have you altered any of the three cats? Are the pre cats removed (headers installed) or gutted? Do you still have the original main cat or did you swap it out for a high flow cat?
Also, were the cats propely warmed up prior to the test? Not just a well warmed up engine, but warmed up cats. About 15 minutes of spirited driving immediately before testing often does wonders to test results.
Sometimes if the cats are coated, it helps to run some injector cleaner or something similar through them along with some spirited driving. Berryman's from a "vacuum suck" often brings lazy cats back to life.
Good luck with getting it resolved.
