The only singles that you are likely to find on the shelf at truck tyre places in Europe are 385/65 on 22.5 inch wheels, as these are commonly used on the steer axle of heavy trucks. I suspect that the same is true in the US, although I saw lots cement trucks with huge 22.5 tires (425 or 445 wide). I don't know if the load capacity of the 385s is enough for the back axle of your truck, you'd need to check.
20 inch wheels are not a common size anymore outside of military and specialist trucks, and so getting tyres is likely to require ordering them in, wherever you are. They come in a better choice of off-road treads, and if you want tall rubber, there's not much bigger than the 16r20 XZLs.