The fault codes on this truck have nothing to do with the transmission, as long as the engine is running it should drive. Run 12v pos & neg from the battery straight to one of the trans solenoids (unplug first), see if you have drive, if so the trans is fine.
Swap out the headlight relay for the creep speed relay on the relay bank under the bonnet. Creep speed relay (incorrectly named but what can you do) is part of the park brake circuit. Could also be feed to the fwd/rev lever assy or a faulty OSS under the dash by accelerator pedal.
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