Are there "Hydra-Matic" emblems on the two sides of the hood?
Do you have 1/2 ton truck?
Is it a short-bed or a long-bed (do you know the model number - it is on the ID plate between driver's side door hinges).
If you have an original 1953 GMC Hydra-Matic, I am pretty certain that the ignition must be on in order to "free" the transmission in neutral. There is no Park on that transmission and it locks up in neutral when the key/ignition is off.
Regarding trouble-shooting that transmission, hope that someone who has first-hand knowledge will post advice.
Your transmission is very similar to the one used in Chevrolet trucks in 1954.
