Changing or adding oil in differential or and other gearboxes such as transfer cases without removing hard to reach, stuck or frozen fill plug.

Use oil rated pump and fill gearbox from drain plug hole preferably with snug fit hose end or adapter to the specified manufacturer amount. Wearing rubber gloves and safety glasses have plug bolt/nut in one hand and place next to the drain hole while pump hose end still inserted, remove pump hose and quickly replace plug bolt into drain hole and applying pressure line up threads by screwing in the drain plug. Note: sometimes it helps pumping in slightly extra oil if the plug is difficult to reach and some oil spillage or loss is expected. Also be sure to have something placed under the working area to prevent any leaked oil from staining/soaking floor area.