Rear Diff. Filler Plug!?!?!

Tonight i figured it might be a good idea to change the fluid in my rear Differential seeming thats its an easy job. took it all apart got the old gasket off, cleaned it all up and put it back together only to figure out i cant get the damn plug out!:eek7: i tried and tried but gave up not wanting to strip it out, i tried letting some WD-40 sit on it but did nothing, i did clean it out to re-insure the 3/8 extension would go in as far it possibly could, now its 1AM and i have no car to get to work in at 8AM, also when i put the new gasket on i did not use any sealer, is that going to be ok? i made sure there was no signs of the old gasket on there so i think it will be ok. if anyone could please give me some input to get that think out it would be much appriciated as i need my car!

Tonight i figured it might be a good idea to change the fluid in my rear Differential seeming thats its an easy job. took it all apart got the old gasket off, cleaned it all up and put it back together only to figure out i cant get the damn plug out!:eek7: i tried and tried but gave up not wanting to strip it out, i tried letting some WD-40 sit on it but did nothing, i did clean it out to re-insure the 3/8 extension would go in as far it possibly could, now its 1AM and i have no car to get to work in at 8AM, also when i put the new gasket on i did not use any sealer, is that going to be ok? i made sure there was no signs of the old gasket on there so i think it will be ok. if anyone could please give me some input to get that think out it would be much appriciated as i need my car!

They can be a pain if never removed to chek oil level or installed to tight.
Put a big long breaker bar on it holding in on your extension.

If it strips out inside you willm need to drill it out and install new plug and always pull cover off and clean out metal shavings inside.

Dry gasket should hold but a little sealer is better.
I like the 3M yellow weather strip stuff on paper gaskets.

Good luck