After World War Two, the U.S. Military surplused all kinds of equipment from jeeps to trucks to tanks. Many Skagit yarder hoists had been
mounted on wood sleds and were being used with rigged trees- but this surplus of equipment made it affordable for many loggers to re-mount
thier yarders onto trucks or tank-hulls, much easier and faster to move than the old cumbersome sleds were. Here's a great example of that.