Did anybody miss the part where he said it's a 5500? This isn't a pick-up, it's a medium duty truck. The axle isn't too small (13,000 lb. capacity). It isn't over worked. It doesn't have minimal oil capacity (nearly 5 quarts). This is a commercial vehicle, not Johnny Contractor's daily ride.
My first order would be to ask when the last time the rear brakes were pulled apart and inspected? Any signs of a leak at the inner bearing seal?
The outer bearings in a floating axle support the full weight of the vehicle. There is a pool of oil that partially fills the outer bearing housing that lubricates the bearings. If the seal leaks, the pool gets emptied. The pool is filled by gear oil traveling from the pumpkin, down the axle tube, and into the bearing housing. If enough fluid is lost, fluid will only travel down the axle tube when cornering and eventually, not at all. This leaves the bearings high and dry.
I have seen chassis cabs get brought in on the hook with the bearing welded to the spindle due to a lack of lubrication. There are actually mobile repair guys that will cut off and weld new spindles on for this exact conditions. If there's enough demand to support a mobile repair guy, it's not all that uncommon.
Thing is, most rear brake inspections consist of peeking at the pads, looking at the material remaining, and calling it good. Unless it's leaving puddles, nobody looks for a leak and nobody checks the fluid level of the diff. Many bearing seal leaks get missed simply because nobody ever looked for them.
