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First off, you will need to know your axle ratio. The door sticker should tell you what axle you have.
My truck is a '91 F350 with a 351, C6 and a C9 axle. The C9 works out to a 3:55.
My owner's manual lists my truck at 6,900 trailer with a max GCWR of 11,000
If you can get me your axle code, I can let you know what my manual says (it is for 1991 Ford trucks)