Replacement axles for our vintage Airstreams is a bit more difficult than in the past. As far as I know, Dexter is now the only manufacturer. Dexter has distributors all around the country, but they are not necessarily familiar with Airstream trailers. Airstream dealers parts department may be able to help. I don't know because I have not gone that way before.
Keep in mind vintage Airstream axles are special order for Dexter. There are no refunds for order errors. I needed an experienced person to help insure they were right and take responsibility for the order.
I have replaced old axles on three trailers over the years. Colin Hyde Trailer Restorations in NY state is a recognized Airstream axle expert. Inland RV in California is another expert. These folks certainly can help you through the process. Here in Colorado we have Luke Bernander at AirstreamInCo@gmail.com
I've ordered two sets of axles from him with good success.
Items for your consideration:
Weight carrying rating for each axle
Mounting bracket outside to outside dimension
6 bolt on 5" circle wheel mounting
10" drum brakes standard on 3500 pound axles or less.
12" drum brakes standard on 3600 pound axles or more.
Swing arm starting angle.
Hub to hub face dimension.
Airstream shock mounts welded on.
High profile mounting brackets wanted (about 1" higher)
This list is a starting point. I don't know the Dexter order form by heart by any means.
Here is a photo of the new Dexter axles we just took delivery of last week. We're renovating a 76 Sovereign
31' similar to your trailer, but different.