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With 3.55 gears it's rated for a GCWR of 15K and 18.5k with 4.10's. GCWR is the total weight of truck and trailer combined. This is right from my 1990 F-Series owners manual.
My book says the 5.8/auto/3.55 combo is good for 11.5k and 4.10's will do 13k. Tow ratings are based on the powertrain combo only and that means the heavier your truck is the lighter the trailer has to be to meet the combined weight rating.
Your individual truck needs to be weighed. Tare weight would be unloaded, full fuel tanks and no people.
Then add your personal stuff, your people and you normal load of stuff in the bed. That would be the total weight of your truck as it is going down the road.
Whatever the difference between your loaded truck as it is going down the road is what you can legaly tow.
Your individual loaded truck weight might vary from the next by 100 lbs, 500lbs, 1000lbs or more. I am just trying to make the point that you have the absolute GCWR from Ford, it is up to you to figure out how that works out between your truck and your tow.