CAREFUL WITH THE IMPACT. I think the situation you are running into is similar to mine on an old Muncie trans. I when to change the diff lube in it as well and the steel plug had pretty much welded itself into the aluminum case. I tried everything, which led me to the impact. 30 seconds of impacting and boom, sheared the plug hex right off. Had to drop the trans disassemble it drill it out helicoil it, not fun you get the picture.

__________________
2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.