Apparantly I have.. I tow a 25' boat. It's a little slow up the hills boat the truck runs cool. Try not to let the trans change gears to much. There is a sweet spot that you will find.
I also to a 21' v nose trailer 101" wide loaded with 3 quads 2 dirtbikes and camping gear for 5 people, again no problem.
Electric brakes are a big help and keep the speed to 55-60
For short distances, that weight will be okay. I'm thinking you'll want trailer brakes for that kind of load. I've used the FJ to tow some heavy stuff and it's never fun. It has a hard time. I think it's a combination of the wheelbase and the gearing. If you have an AT keep it in 4 when towing a heavy load.
I towed my TJ with my FJ pre SC and it has to weigh over 5000lbs with all the extras in and on it. It did pretty good certainly NOT ideal or something I would buy to tow 5000lbs but the FJ did the Job. If its going to be a regular event I would buy a f250 or at a minimum get a better load bearing hitch set up and brake controler for the FJ. A SC would help too. I towed 225 miles 800 low elevation to a high of 1400 nothing to bad
i pull this set up all over just went to hot srpings ark from south louisana 370 miles one way 65 mph 2600 rpm 11-13.5 mpg trailer brakes a must , and keep it in 4th gear in reg D it wants to search to much .its a 17 ft trailer