Where do rear axles get heat from and what causes them to warm up if there just a final drive?
They produce heat from the friction from the gears meshing together while running down the road.
The span from the ring gear case to the wheels produce a small amount of axle twist when force is applied. Any time force or twist is applied it creates heat within the axle. They're enclosed in a third member housing where hot oil from the ring and pinion can leak buy the inner seals. Cheap light-weight trucks and cars don't have third member seals so the hot ring and pinion oil will splash down the outer axle tubes.
