Update to help people looking for the same issue:
I fixed this by investigating connections and wire harnesses BELOW the dash near the gas and brake pedals. This turned out to be loose connections to the GEM (Generic Electronics Module), which looks like a 2nd fuse box. Re-seating all cables to it did the trick!
I actually got tipped off to this by an Ambulance mechanic in the area who works on rescue vehicles with Ford E series front ends. From now on I will include Ambulance mechanic shops in my resource list for wherever I am travelling!