With this a recurring problem, apparently difficult for a shop to diagnosis, I’d probably just swap the two rear wider rear tires with the same size of tires it came with from the factory and do a drain/refill of the axle oil, making sure to include the correct amount and spec of additive per the owner’s or service manual. Then just monitor the situation. I’m guessing there’s a good chance that will solve it.
Beyond that, you already understand I presume that there’s only so much force the components in that area can take. If this is a work vehicle, and maybe you are a roofer, so you carry a lot of weight, that’s the reason for the wider tires, then earlier than normal failure may just be something you have to live with. The Ford 9 inch differential is the one the folks who build race cars seem to like, b/c it can handle pretty much any force thrown at it; if your truck is doing severe service, if this happens again ask you shop if it is possible to switch to that one.