Three problems related to differential unit have been reported for the 1999 Ford F-150. The most recently reported issues are listed below.
I heard the rear end make a horrible loud bang on several occasions till I finally had no ability to make the vehicle move forward or backwards while in gear. Upon inspection by a certified mechanic, a main bolt on the rear differential undid itself and stripped all of the teeth on the main gear. A complete rear end had to be put on. There was never any work done to the area of the vehicle prior to this incident.
Noise and bump in the rear diff from a stop or turning.
The rear end of the vehicle made alot of noise. A TSB was issued for similar vehcile related to differential chatter.