Really depends on what kind of service was performed on the diff. But I am saying your are incorrect.
2018 to 2020 is more than a enough time for something to go bad on its own. A bit of a stretch to cast the blame now.
Do you have a copy of the service ticket? Reason I ask is that sometimes a tech may write a reccomendation down on the ticket while performing the service you requested. But often, it doesnt get back to the client.
Example being; you have a rear cover leak serviced. The tech notices the pinion seal is also leaking. He Writes it on the ticket, the service writer overlooks it, and hands the keys back to you without any mention.
I find it interesting that you used the word, “stealership.” It seems as though you alrrady have your mind made up in that they caused the issue and want others to validate your same view. No need to play victim, find another shop to take it to that you feel confident in. If in your mind they didnt fix it the first time, why would you take it back?