I had to WD40 the top 2 bolts several times. Alternating with hammer blows
Got all the bolts out, no problem except for the top 2. I used the WD40/hammer method to get them out. Lube, tap, lube, tap, about 5 times until the rust broke free enough to remove the bolts. There was substantial surface rust on the gasket surface. I covered the opening and wire wheeled it then used brake cleaner to clean the surface. Went to Advance to get new bolts because the originals were in poor shape. The brake line bracket on the right broke but the other one is in good shape. I put the new cover on and filled the differential in the rain.
Heat is your friend here. Heating up those bolts with a small Bernz-o-matic torch like plumbers use would likely have made short work of removing those top two bolts. I always keep a small torch handy for issues like that which is easy to get into tight spaces. I also have a full blown fire wrench (oxygen/acetylene) rig for when it's time to get serious.
Glad you had success nonetheless.