This is what AI says about it. Looks like you can swap the selector switch, but it also recommends to swap the other wiring and solenoids inside the transmission.
Do not try to force the 1999 truck's wiring harness into the 1996 transmission plug, as it will not scale correctly to your 1999 computer.
Before lifting the 1996 transmission into the 1999 truck, swap the transmission wiring harness and the internal solenoids from your old 1999 transmission over into the 1996 transmission casing. [1]
- Drop the transmission pan on both units.
- Carefully unplug and transfer the hard internal plastic/wire harness wrapper and the solenoids from the 1999 valve body to the 1996 valve body.
- This ensures the 1999 truck’s computer gets exactly the electronic signals it expects, making the transmission fully plug-and-play. [1]
Additionally, make sure to swap over the Digital Transmission Range Sensor (neutral safety switch) on the shifter linkage on the outside of the case, as the plug design changed between 1996 (8-pin) and 1999 (11/12-pin)