I have an old '83 F150 w/ a 4.9L, C6 and 3.55's. Once, I towed a twin axle U-haul car trailer 150 miles empty and it did fine. Came back towing a two door Ford Explorer that had a dead tranny. Pulling a grade was 'hard' since I could only hold 45 on the interstate (hills and pass). I've towed a JD 2640 tractor (5000lbs tractor weight) so I know it had to be about 7 or 8K. I could hit 50 no issues. Get up and go was an issue, but after you got moving, no problems.
A friend had a 88 F150 4x4, 4 speed with 4.9L and 3.08's. Yes, a 3.08 ratio. He hauled a two axle car type that was loaded w/ veggies for a fruit stand, plus his truck was loaded. The trailer pushes 3K empty (custom built multi purpose). I'm guessing his GVWR was 9-10K.
A teacher had a early 80s F150, 4x4, 4.9L 4 speed w/ I assume a 3.55. He had a full size slide in AND towed a boat behind it (one big enough to water ski with). He said it did fine and even wishes he never sold the thing.....
Both my 90 and 99 diesel vans have 3.54 and 3.55. I plan on putting hitches on them and I figure that I could tow fairly well with them.