These trucks can tow WAY more then then what they are rated and there is 2 critical reasons why.
Full military wraps on the frame side and half wraps on the shackle. As a mechanic I've only seen logging trucks and heavy equipment have this. If your primary leaf breaks from the load, you have time to save it and limp home. This is the only reason I still run toyota springs on my rig.
The torsion bars are on the top of the frame and run through both sides of the a arms. Again this is something I've only ever seen on heavy equipment. Toyota ifs is one of the strongest ever built, I've yet to break mine or damage it in anyway. The truck gets jumped all the time and slammed over medians when I want to skip the light.
Here's an example of a recent load I hooked up to. Btw I built my own receiver hitch, it's made out of 3/8 plate and 1/4 tube/angle.
I pulled a 8.5x16ft twin axle mobile home trailer weighing about 9k. The trailer had no brakes and I was able to get the suspension off the bump stops with a weight distribution hitch and throwing all my tools and some cinder blocks behind the trailer axles.
It had more then enough brakes, it wasn't even scary. I could get up to 50mph on the flats, and starting on the hills in Seattle I didn't have to be in low range but I did just to save the clutch alittle. In low range it barely felt the trailer. I had to pull over and take it out of low because I was holding traffic up. On the really steep hills I couldn't get out of 2nd and would turn the engine at 4k rpm with only the primary barrel open.
Oh, I got 14mpg doing it too. I took the trailer 40miles in 1.5hrs from shoreline through downtown seattle to kent. I burned just under 3.5 gallons.
92 toyota pickup 22r, weber 32/36, 4.88 gears, 31x10.50 tires. Engine has almost 300k on it and runs great not burning any oil. No sway bars or load leafs. Electric fuel pump and fan. 160 thermostat. Lots of exhaust leaks. Used a 22re for the high compression ratio. Weber is modified with a steel plate so it can run at any angle.
Pound for pound these trucks are the toughest ever built. You just got to man up and beat it the **** up, she'll get it done.