NOx is caused by high combustion temperatures. Normally inert nitrogen in the air is under so much heat with oxygen present that it binds with the oxygen to form oxides of nitrogen. A faulty EGR, too much compression, bad cooling system, and over lean mix will cause high NOx levels. High load at low RPM can also cause excess NOx. I always recommend a good tune-up and de-carbonizing procedure be done on the motor (jiffy lube can do a cheap version of it). A Chilton should have all the procedures needed to test the EGR valve.

Do you have over sized tires? What are the other numbers (CO,HC) at 15 mph?