The most common truck fitted with an Eaton two speed Axle (from an Australian Military Vehicle point of view) was the Dodge T110-L5 (3 ton Dodge MCP). I'm pretty sure that the one bought by a certain SA collector from One Tree Hill had one of these fitted 'cos I had a good crawl around it before it went west.
Eaton Two speed axles were fairly common on Commercial trucks of the '30s, '40s and '50s and were also available of Fords, Chevs and Inters and some were fitted "Aftermarket" to a variety of British trucks. Some of the MCP Ford and Chev 3 tonners got them, too. I'm actually surprised that they weren't considered in place of a transfer case for Carriers or the Tracked Truck. They have a high ratio somewhere inthe vicinity of 6:1, reducing to 8:1.
Keefy might still have some of his Dodge T110 service manuals to show how the Axle selector worked in conjunction with the speedo reducer.