I have seen that on a 6.0.
Damn this truck makes me crazy
I finaly found the EGR cooler was bad by using coolant dye.
I road test the truck hard and remove the EGR valve.
With black light, i have found coolant presence on the valve.
It may be possible on the 6.4 to,but wich one of the 2 is fail ???
Maybe if you try to heat the cooler a bit with the gaz wrench