Have voltage readings been made from G to N / L to N / L to G?
What happens if the GFI Receptacle is pulled out of the box and the ground wire is removed from the receptacle?
Since all of problem pedestals are fed off the same main it seems like the cause is most likely back at the main disconnect.
But then again is could be a mis-wiring at any of the pedestals.
Is the feed to all of the pedestals in conduit the full length? If it’s URD maybe one of the wires are damaged.
