The contact owns a 2017 Ford F-150. The contact stated while driving 80 MPH, the transmission unexpectedly downshifted from eighth gear to first gear. Additionally, the vehicle jerked. The vehicle was driven to the independent mechanic where it was diagnosed with transmission failure. The contact was informed that the transmission needed to be replaced. The transmission was rebuilt, and the vehicle was repaired; however, the failure reoccurred. The contact stated that while driving at 35-40 MPH, the transmission was in tenth gear. No warning light was illuminated. The vehicle was not diagnosed or repaired by an independent mechanic or the dealer. The dealer was made aware of the failure. The manufacturer was not made aware of the failure. The failure mileage was approximately 135,000.