I had a '06 with the 4.0 liter with 6 speed manual and towed a Searay 230 Sundancer. The weigth of the boat and trailer were about 6300lbs which is right at the tow limit. I towed it from Calgary to Vancouver a few times which crosses over 3 major mountain ranges. On a hill that is a 8.5% grade for almost 20 km the truck made it with no overheat issues at all. I could only maintain about 70 km/hr on some sections which was alot better than a friends early 2000's F-150 with a 5.4 and a lighter boat.
After towing that a couple of times I decided to upgarde to a Tundra which made things so much easier that I did the only logical thing, I bought a bigger boat.