The selector valve is under the driver's side and mounted to the frame rail. It will be below the driver's door. (See pic 1 below)
As far as what you described, it could be a faulty check valve in the rear fuel module. That check valve prevents the fuel from being fed into it.
Let me know if you find the selector.
Image (Click to make bigger)
Tuesday, October 19th, 2021 AT 8:33 PM