The pic above is from the shop manual I have. I have see nothing referring to a locknut. It has worked just fine since about 1990 just the way it is. I was just thinking maybe something in the trans itself has changed or become worn maybe. The pic above looks identical to what I have plate, snap ring and switch.
Eddie,
81 F100 Custom SWB, 5.0L, 4x2, Single Rail Four-Speed Overdrive, 3.00 Non-Limited Slip Rear Axle, Non A/C truck to complete 85 F150 factory A/C conversion, Nutmeg interior color.