If you’re not pursuing big power nor running big tires I’d stick with the stock axles and gearing. I’ve owned my Series III for almost thirty years with a stock driveline and never had a differential or front axle failure. After a rear axle snap early in my ownership, I replaced them with Great Basin strengthened 10 splined shafts which solved the problem. I am not sure if Great Basin still sells this product but other retailers still supply a strengthened option.
My usage was loaded, overlandy type stuff with mostly moderate off roading done with a lot of mechanical sympathy. If this is what you anticipate doing - building up your existing tubes with parts from any of the any parts suppliers should be easy. Trevor, at the mentioned Rovah Farm, saved my bacon on a recent trip. Rovers North often lists used diffs available. Etc.
Henry