As Bruce mentions, my money says 4 wheel drive on a hard surface along with incorrect tire pressures. It causes driveline wrap up, and will overheat the gearboxes, making the oil have different properties.
On the old 5/4 ton, if you drove in hi-lok at high speed, the front diff (which seemed to be the weakest link in the drivetrain with it's puny dana44 diff) would boil the hypoid, causing it to spray out the vent tube into the engine compartment.
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