The weight limit is determined by the weakest link in the entire system, so please don’t go just by the rear axle load. Since you have a relatively light 24.1 floorplan, especially if built on an E-450, for example, you may overload the frame rails themselves and or the frame extensions before the axle is overloaded.
Additionally, hanging an 800-pound ATV well behind the rear bumper is much worse than simple down force from a trailer ball that was located within a foot or so of the bumper.
Thor and or Ford engineers may have also placed that limit for reasons that are not obvious.
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