My 42 (now owned by JLucas) had a mechanical linkage, which had a cable that went from the linkage rod to the rear end. The cable was VERY stiff and hard to shift. One day I could not get the cable to move at all so I rigged up a solid linkage, which worked great. I used the clutch to shift the two speed on that truck. BTW it has a Timken double reduction axle.
Most later trucks I have driven had either electric or vacuum two speeds. I would use the clutch when shifting the axle at low speeds, for instance, split shifting when hauling a heavy load of wheat out of a soft field. I would not use the clutch when shifting the axle at road speed.
