If the goal is simply changing the diff oil and you removed the oil with suction, why would you need to worry with the cover? Just fill it back up.
I would be surprised if you could even pressurize the diff to get the cover off. Seals would probably blow first then there goes your pressure.
I usually just hit mine with a rubber mallet. breaks it loose enough to easily pry it the rest of the way off.
77 CJ-7- AMC 360, Edelbrock cam, MC2100, Fenderwells, CJ T18/D20, Chevy D44/EB 9" rear, Trutrac f/r, 4.11, SOA, 1.25 lift shackles, 35x12.50 BFG KM2s, Shackle Reversal