Does a sub-panel in an outbuilding require a ground wire or rod?
Larry, I can relay what is required in our area.  For each and every premise now is allowed one service panel only; thus requiring only three wires to feed.  All other panel on the premise are required to be fed, for the sake of safety, with four wires.  These include two wires as phase conductors, one neutral and one ground. Additionally, all panels that are  located in a separate building require ground rods, no longer just one ground rod but two separated by 6'. In the service panel the neutral and grounding all come together at the neutral bus bar.  The subsequent panels must have all neutrals and grounds separated. Good Luck; PCL