Exceeding RAWR (or any weight rating)......means more stress which can result in increased wear/tear. Possible axle bearing failure, pinion/ring gear increased wear, stripped axle spines.
Are axle ratings downgraded by vehicle mfg.? YES.
Axle mfg. ratings are higher then vehicle mfg. ratings assigned to the axles because of suspension components (tires, springs). Vehicle mfg. rate the whole set up where as axle mfg. rates the individual component.
So......
How much can you exceed vehicle mfg. ratings without causing harm?
About as much as the rear tires can handle.