As others have noted, the axle leaves the factory with and upward bow. I've been through this situation three times over the last fifteen years. In each case it was the rear axle and it was damaged, (as in bent out of alignment) by striking something, pot hole, road debris, etc.....
There are some here who claim that alignment shops, trailer repair places, etc... can repair and or realign it. In two of my three, they were under warranty and both manufacturers claimed that the only solution is the new axle that they paid for. In the third case I took it to a truck suspension shop who claimed that they could repair it. They looked at the tire wear and told me that the axle needed to be replaced. hope you find a cheaper, better solution, but having been down this road a few times, so far I haven't been so lucky.
