Jolly Roger I am not familiar with a Wayne body with a Thomas chassis with an 8.2L. In that vintage with that engine I would guess you have either a Chevy or a GMC chassis under your Wayne body.
Finding a good rear end with the gear ratio you want to fit your rear end may be a problem. GM used some OEM vendor supplied rear ends and some of their own. The OEM vendor supplied rear ends might have some available choices for you. A GM rear end will be hard to find since GM sold their medium duty line more than ten years ago.
In your case it will most likely to be easier to find a good working rear end with the correct gear ratio and swap out everything. It may require you to go to a 10-hole Budd wheel. But if you are purchasing a rear end from a junk yard you should be able to purchase the wheels and tires to go with it at the same time.
If you have 9.00x20 tires it means you have tube type tires with a multi-piece rim. Regardless of whether or not you need to go to 10-hole Budds or not, you do need to think about swapping out your tube type tires for tubeless. You will have a lot more choices in tires with a 10x22.5. You will also pay about as much for a new 10x22.5 tire and a good used rim as you will pay for a 9.00x20 tube type tire, tube, and mounting them on a multi-piece rim.