I know this is an old post, but I tried to find the most recent one on the subject. I pulled trailers for a living. Heavy 18' trailers with 3/4-ton standard cab trucks on the interstate and off for a route type of job. I also drove an 18-wheeler flatbed for a short while. I knew how to back a trailer before I had a permit to drive. I think I've established enough experiential credibility.
My dad bought a 1974 FJ40 from the dealer brand new when I was 2. It was the only car he ever bought brand new. I still have it today. We had a 14' x 4' trailer we used that we would pull behind it for camping. We also had an 85-horse fiberglass boat we'd drag to the lake about an hour and a half away with no problems. Dad kept the steering tight and when he and I (my 4 sisters were NOT allowed to help) loaded the trailer we balanced it carefully so the weight was at the right proportions. In my employment we'd do the same thing whether it's an 80,000lb semi or a 3/4 ton with a John Deere and a ton (literally) of seed on it plus various other equipment. It all had to be loaded carefully or you had to fight the rig at any speed above 45 or so.
While the 2F doesn't have much top end so you do have to let it grunt to pull heavy stuff, that is one question. The question of how much can an FJ40 pull? We've pulled a 9-ton trencher once that had surge brakes. We had to drop to low range at each stop and then go into high range after 4th+low. Then to stop he'd shift down until he got to first + high since you can't downshift the transfer case. It wasn't relaxing ... at all, but we did it. How my dad talked the company into letting him pull the thing I don't know. It was a different world ... the lawyers hadn't wrecked it for us yet.
Many have said the '40 is light. LOL compared to what? Not compared to a Cheep. The axles are both 3/4-ton size. If you swap to a full floating rear axle from an '80 you're set to carry some real tongue weight if you put some helper bags above the axles. They help with that factory squat and list anyway. If you've got multi-link in the rear ... I can't speak to that. I have zero experience there.
As many are saying, trailer brakes are important. Also, as has been said it isn't hard on the '40 or the trailer. The ones I installed on my trailer were simple magnetic brakes. Seems like those might do better in salt water than more complicated electric over hydraulic (if there is such a thing??) An empty '40 weighs around 3700lbs. Put gas, your peeps, and the ice chest in it and it's gonna take on weight like I did when ... pizza went on sale. The point is try it before you tell someone it isn't safe and the '40 can't do it. She's a beast that can do a lot more than most think. Especially if you flip your springs like most do to get a 100" wheelbase. Just chill with the "can't" stuff. The '40 was designed and built for American soldiers in the era of "can." That never changed.