With no metalic paths I think you are better going 3 wire and regrounding the neutral ... for all the same reasons we ground the neutral at the service in the first place.
Sorry Ryan
Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
