I have never seen a actual spec sheet from the manufacture with these ratings. Not saying it doesn't exist, I've just not been able to find it previously. I've heard numbers ranging from 10k up to 11.5k for the axle rating. I think that an important consideration with this is, even if it's true that the axle is rated to 12k (for sake of argument), there is no way of knowing if it's "up to" 12k, or a static 12k. Not every OEM is going to purchase the top level spec of every item from the manufacturer, especially if it's not needed.
Past that, I've always been told that the GAWRs are to include wheel, tire, and suspension, not just the axle rating. So while your axle may be capable of supporting 10k in the default configuration, it's guaranteed that your tires are not, just as a single example.
Again, this discussion is about as old as time. Some guys want 50% overhead on their payload/GAWR numbers. Some guys are comfortable going 50% over. Some guys don't even look at the numbers and say fun quotes like "don't worry about the mule, just load the wagon".