Unless your Toyota 4Runner is equipped with an engine size other than a 3.4 liter it will not be equipped with an EGR valve, or EGR system. NOx reduction will be primarily controlled by proper air to fuel ratio, and your Toyota 4Runners catalytic converter.
You'll want to ensure your 4Runner is not running lean, has no vacuum leaks, engine compression is not high, and the engine is not running hot. NO is created when combustion chamber temperatures rise over 2500f. Any of the conditions mentioned above can cause engine temps to increase, resulting in high NO.
The diagnosis for proper air/fuel ratio will include a "fuel feedback test". This includes ensuring your Toyota 4Runner's oxygen sensors are working properly, the ECM (engine control module) is receiving the signals from the oxygen sensors, the ECM is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers. At the end of the feedback test it will be know whether there is a fuel control problem causing high NOx, and whether it is electronic or mechanical. The technician can further diagnose the fault. There will be several other emission sensors which will be inspected during the feedback test as well; including the MAF (mass air flow) sensor, ECT (engine coolant temperature) sensor.
