From a quick look the axles I'm not sure that's the original rear axle. Comparing it to my '41 3/4 ton. Mine has a plug in the rear cover to check oil level. Can't see it in the photo but there's also a couple bolts that locate/hold the pinion bearings in place. Your rear has been converted to tubular shocks. The springs have been modified, my springs are mounted on top of the axle where yours are under. Mine is the lighter duty 3/4 ton and has 7 main springs and 6 overload springs where yours has 2 leaves total. Your front springs appear close to original. I'm not a suspension expert but I would suspect you have very different spring rates front to rear. First easy check to do would be to check the gear ratio, should be 4.557 if original..
