I am putting a new diff in my car. Well new to me and I went to pop the plug to fill it off and it wont budge. Also the ratchet just wants to strip out the plug. Has anyone had this problem? I need help despritly.

 

WD-40 the hell out of the bolt on the sides where it is screwed into the cover... let it soak for an hour or two (to eat up the rust) (depending if you have lots of rust) and try it again...
even if it looks pretty clean of rust... let it soak for a while.... That should do the trick

 

pb blaster is abou the best penetrant you'll find, try it. a little heat will work too.