Have Ford Questions? Ask a Mechanic for Answers ASAP
The problem is the speedometer in your F150. The VSS sends the signal to the speedometer and the speedometer sends the signal to the PCM (Engine Computer) which controls the transmission shifting. You can either have the speedometer replaced or repaired by a speedometer repair shop. It is actually and small module called the PSOM (Programmable Speedometer/Odometer Module).
What else have you replaced or repaired to fix this problem so I do not duplicate anything.
If I remember correctly, where the speedometer cable connects to the transfer case, there is an electrical connector there also.
That is the VSS for the transmission and the speedometer. You may also want to inspect the speedometer cable where it connects. We have seen them twist and also break back in the 1990s.