There are no 2 speed axles available for 3/4 or 1 ton trucks. There used to be a 2 speed gearbox built by US Gear but they were bought out and production ceased with all spare parts destroyed. I had one behind a 454 and Turbo 400 3 speed transmission. It was really a robust gearbox and was electrically shifted allowing overdrive in all 3 forward gears. IMHO the US Gear was far superior to the present day Gear Venders. The only available 2 speed now is a Gear Venders over/under drive and they are available for 99% of all transmissions and engines. Modern day 6 - 10 speed auto transmissions and high torque diesel engines have killed the need for 2 speed axles or gearboxes.
https://www.gearvendors.com/index.html