I wasn't aware that 1-1/2 tons had enclosed driveshafts. If you have an open driveshaft you can put an auxiliary behind the main transmission. I did this many years ago and am still driving the truck. The one to look for is a Spicer or Brown-Lipe 5831A. These were found in many tandem axle trucks, dump trucks are most common in my area. Or the next bigger transmission would be a 6231A. There is a fellow who advertises in the ATHS magazine who has at least one of these.

The "A" designation is important as those have the 1.24 underdrive and 0.86 overdrive the other have a 2.xx underdrive. Those are no use on the road; the under is just for crawling around on construction sites or farm yards. So you only split the main box gears twice rather than 3 times as with the A models. It makes a big difference on the road to have the extra gears.

You could also put a two-speed axle from a later truck but they were usually geared pretty low to accommodate 9.00 X20 tires.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.