Oil was dirty but not that much. I have no idea when it was last changed. I had some mud on the bottom that I will remove. That mud, is a little bit greyish but really light. And no metal bit !
I will see how I can drive the spacer in place.
I checked with RockAuto and they do have Brake & Wheel Hub : Wheel Seal and Drivetrain : Axle Shaft Seal
Seems to be the same part?

 

That "mud" is years of accumulated worn off metal from the internal parts, it's normal. You will find that stuff to be magnetic. Is there a magnet attached to the inside of the diff cover? If not, you can add one.
The second link is the rear axle seal that you want, not the (front wheel hub) ones in the first link.

 

The first link from RockAuto list well 2 Rear (and Front) Wheel Seals, that is on their "Wheel Seal" section. In doubt I will get them from their "Axle Shaft Seal" section...
No magnet on the differential cover.

Any tip to open the square oil fill plug? I heated it to 300°F with no success and the 9/16" wrench wants to get around the flats... Can we get a square bit for a shock wrench?

 

Do we need a gasket between the bracking plate and the rear axle? I can't find any with vendors and did not had any when I removed the plates.

 

Been decades since I had one apart,I believe the axle seal is all that is needed.
​​​​​​​I don't recall a gasket being there but you could buy gasket material and make one or just use a bead of silicone if you want one.

 

Thank you
Not that I really want one, just want to use one if it's designed so. It'll stay naked then.