Rear diff started leaking. How stop it ?

Check the bolts make sure they are tight. Lots of times internal diff problems first symptoms are not being able to keep the bolts tight. That said getting them tight as possible and keeping check on them will buy you lots of time

 

You’ve probably already sealed it up, But get a tube of seals all, in the yellow tube, It’s cheap, thick clear, dries hard, it will seal gasoline, I’ve fixed 2 gas tanks with it. Great stuff, good to have in tool box.Stands up to solvents.

 

I cleaned the breather and put Lucas in there. the leak is slow now. one drop every 10-12 min. I will keep watching the level until I 'll get home. Don't want to try to seal it because as soon as I get home I will properly fixl it.
I was unable to find 2-3 quarts of synthetic diff oil in truck stops and nearby part stores. I called two places yesterday and they said only 5 gallon buckets are available. Decided to go to Autozone. Autozone had Delo Syn-Gear XDM SAE 75W-90 . it is O-76 spec extended service diff gear oil. One quart was $7.