I spent 600 dollars a total of 3 weeks in the shop to get new plugs wires trans/converter fluid changed,all in the last 8 months. I took it to an independent trans guy who listened and test drove my truck carefully, then spent a few minutes checking under my truck to find that my trnasfer case had 3 loose bolts.
My trucks symptoms were a shudderring at 40 mph until it shifted out of gear to a higher one. It also would repeat this on acceleration above 45 mph. Fors dealers all liked to blame my K/N filter or the Flowmast dual exhaust. They refused to believe me when I insisted something was wrong.

My mods are flowmaster dual exhaust
K/N filter charger