used to sell replacement axles but they are all sold now.
Your best bet is to have one made by Dexter or Flexiride or similar. You provide the dimensions and they are all custom made. This would be a great opportunity to add brakes
Remember that the stock UHaul
wheels have a large positive offset. You will probably want to replace the wheels with standard zero offset (4.5"x5) wheels at the same time, which will affect the new axle
dimensions. Check the wheelhouse clearances twice before you order.
Dexter Axle - Trailer Axles and Running Gear Components - Canada