NCXLT
an important factor in towing is gear ratio. mine, for example, a 2000 with 3.55's and 1500 lbs behind it will roll fine on level terrain but wants to shift down if you even see a hill in the distance. a head wind will keep it out of O/D entirely. ( i know the book says to tow with O/D off, but i can' afford the fuel to do that) it is rated at "6001 to 7000 GVW". in contrast, my (old) 97 light duty f250 with the same engine/trans. but 3.73 gears had only problems with the steeper hills.
also, tire size will have an effect on final gear ratio.
hope this helps, brian