I'm going to take a contrary view here, I think.
What's the issue?
You're putting far more torque through the axles than the truck was built for, but you are demanding that Subaru cover a failure? I'd have some sympathy for you if there was some kind of evidence that this 'wear' was because both axles were made improperly rather than them failing from you doubling the design-torque through them. And, you're saying that your dealer is willing to work with you on that front.
Absent that, you're coming across as just another whiner who wants to avoid taking responsibility for the mods you've made, and make the price of a new Subaru higher for the rest of us.
What did I miss in this story?