A 3-53 wouldn't give you a whole lot of power than what you have. I have a few air brake axles with two speed rears, but they are very rare. Your chances of finding one are pretty slim. Something like a 12 valve Cummins might work. Single speed rears are everywhere and go for scrap iron price. I still have a lot of junk from when I was a licensed salvage dealer and I still have dozen trucks in my fleet that were built from salvage. The components you are talking about like a 5 speed Eaton and a 4 speed auxiliary would cost a whole lot than you probably paid for your truck in the first place.

https://photos.app.goo.gl/1BkcJZs7egKdn9W19