I take it you are talking about a Pantera ZF installation. Unless your ZF is out of your car the only way to drain the gear lube is with the plug on the bottom of the case that is located in the cover plate for the ring gear! The plug on the driver's side is the fill point and the HIGH mark point for the fill. Some ZFs have a fill plug on the top of the gear box toward the back area, but the full point is determined by the side plug.
Mike
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