Can I add a grounding rod to a sub panel at a distant barn?

I know that sub panels need to have separate ground and nuetral bars for proper path back to the transformer.

However, I have a barn that is nearly 600 feet from my house, built during the two wire days, before the 3rd ground prong.

The grounds and neutrals are already tied together at the circuits within the barn, and from the sub panel only two wires go back to the main service panel.

It hasn't been an issue using lights and receptacles, but when you use something with a load that is not fixed, such as an electric drill or a shop vac, thats when I have received mild shocks here and there.

My question is can I add a $10 grounding rod at the sub panel, and leave the neutrals and grounds tied together instead of burying 600 ft of new wire?