The dry pin weight is 21.5%. If you add 1500 lbs of gear then at least 21.5% of that will be added to the 2580 number. But in reality it will be more due to the location of logical storage in a fiver. It is very possible that 40% of that 1500 lbs will be on the pin making it 3180 lbs for example and very likely over the rear GAWR of 6084 lbs. The axle isn't a problem, it is good for around 10k lbs. Tires, wheels and springs are the limiters. Upgrade all three and you might be ok technically, but you can't easily change rear GAWR and that is the one rating that is legally enforced.