I'm adding a circuit to a subpanel in my garage (built in the late 70s), and I've been reading a lot about subpanel ground / neutral bonding (how it's not supposed to be bonded at the subpanel, and that the ground / neutral need to be separate in the subpanel and you're sending the subpanel neutral back to the main panel neutral).
My subpanel is fed from a two-pole 60A breaker in the main panel, with a pretty heavy gauge 3 conductor wire, two hots going to the bus bars in the subpanel and one white going to the common. However, there's no dedicated ground wire coming from the main panel to the subpanel, and the subpanel's grounding bar goes directly to a ground rod outside the garage.
It seems to me that it's ok to have the subpanel grounded locally, and not run a fresh grounding conductor back to the main panel, but I wanted to double check that. Do I need to pull a new ground conductor from the subpanel back to the main panel? I think this would be difficult if not impossible, since it's about 80 feet away underground and I'm not sure about how good the conduit is.