I have been seeing this truck too much in my shop since we sold it. It's a 98 with a 6 code 4.6. It came in the day after it was sold with a check engine light for egr performance. Well I tested the egr and it was sticking a little so I replaced it. It came back a couple days ago with egr insufficient flow code. I tested the valve again and the sensor too and I could only get 1.5 volts max with on the sig wire with the egr wide open by a hand vaccume pump. It should have been 4-5 volts there. Anyway I thought easy enough just replace the sensor. I did and the code came back as I was backing out of the shop. I really started scratching my head then and thought maybe it was the flowmaster and duals not having enough backpressure to make the pfe sensor happy. I stopped up one pipe and same thing. Then I thought since it isn't on the downstream end it must be stopped up upstream. I pulled off the throttle body and adapter and sure enough the 2 egr ports in the adapter were plugged solid with hard oily carbon.
I cleaned it all out and problem solved.
I just thought I would share this experience incase anybody else is haveing a similar problem. I never in a million years would have thought to look there before trying everything else.

 

Hey m14dan.
That seems to be a problem on the mid to high mileage motors. That is the first thing i look for when i have a low flow problem. The next thing would be the DPFE sensor, have replaced quite a few of them. Congrats on the fix.