Hi
I have a separate garage with a 100 amp sub panel 70 feet from the main house. This panel has a ground rod and currently there are three wires, two hots and a neutral bonded to the rod.
The hots and neutral are run underground separate from the phone and cable in separate plastic conduit lines.

The garage has a plastic water line, phone and cable to it. I understand that there should be a ground wire between the main panel at the house and the sub panel and the neutral not bonded to the ground.

My question is would it be acceptable to unbond the neutral in the garage and run a new copper ground wire inside of the pvc communications pipe (this is the simplest way to run the ground without digging everything up). Is the ground between the panels essential given the ground rod at the garage.

Thanks

You will need to run your ground wire with the other 3 conductors, not in the low voltage conduit. And the bond between neutral and ground must exist only in the main panel.