It could be a few different things but the odds are 99% that it is a bad Differential Pressure Feedback EGR sensor, the sensor that measures the EGR flow rate. The DPFE sensors from that era are not very reliable and many end up getting replaced. Check your dealer because these are sometimes cheaper there than at an auto parts store. Approximate price for the part is $40~60 and they are usually not difficult to change yourself.
Your DPFE looks something like this. It may be metal or plastic.