As egr flow deteriates so does mileage. I don't know much about the sensor.. but I'd check into that as well.
Also, I'm pretty sure it not a problem but.. make sure the passage from where the EGR mounts to the intake air passage is not plugged. On my 4.0L it's very short and I don't think it would ever get plugged. But.. I've seen this happen on MANY other engines.