Split axles are shifted from lo to hi after every shift. Not after reaching 5th gear. However they're usually only driven like this when hauling extreme weight. Being as old as it is this truck should have a vacuum operated differential instead of electronic like the later models. Keep in mind if you ever drive a truck like this, when shifting from lo to hi in any gear you need to pull the handle and release the accelerator pedal for about 1 second for the axle to shift. Then you can get back on it. When up shifting say to second gear from first hi, push the handle in and then shift gears. In order for the axle to shift, wheel speed needs to be lowered slightly. Never shift the axle while going downhill. Hope this helps. By the way, if the axle ever stops shifting right, the first place to look would be the vacuum diaphragm mounted on the differential or the rubber lines running to it. They're very common fail rate items.
you could put it up on blocks to relieve the tires and springs and build a roof over it and use it as a tool shed. may be as time goes on climb under it and get the brakes looked at and clean out the diff and axles.
Have a 68 C40 same issue with brake components. Looking for a Master and booster or just master. I have found ALL the brake parts from wheel cylinders to shoes if your still looking. C40 and 50 have identical brake parts.
