MX ... I did you wrong ... I must have been inhaling kitty litter fumes last night!
I managed to mangle the question in my mind something awful, confusing 'separating the neutral and ground' with 'driving a ground rod.' Two completely different issues.
If you're feeding a panel, you no longer have a 'single circuit,' you have 'feeders.' A detached building requires an additional ground rod (or other grounding electrode).
And, yes, I'd keep the neutrals and grounds apart
