I had this exact problem on an old mini van I had, misfire on acceleration. It was always a loose #6 spark plug that was extremely hard to get to and torque. Everytime it happened I knew exactly what to fix.

That said, this F150s are a different beast but I like the spark plug wire changeout suggestion. I have bought several vehicles where spark plugs leads were never changed and eventually caused misfires or I was down right running on one less cylinder.