Where is the fill plug for the rear differential.

Check out this video

https://youtu.be/v6PjkozhrAc

First look for a 1 1/4" rubber plug on the upper right side of the rear cover. Those are pried out with a flat blade screwdriver. Next, look for a threaded plug on the left side above the axle tube. Most people try to get those out with a 3/8" ratchet extension but those don't fit well, and the ends often twist off. There's a lack of strength there because of the missing metal for the spring-loaded ball. Use a 3/8" square socket. It will seat tighter and deeper than a ratchet extension.

Please let us know what happens.

Cheers