Pulling 6000# is one thing. Stopping is another, and certainly more important. Go through your brakes 100% and maybe consider the Tundra brake upgrade.
Also, these:
Airlift 1000 air bags
Tow 6k…….. with a 3rd gen….wat?……..
Trigger warning, This is going to sound super preachy. I’m late to the thread party, but Just hear me out. I’ve done a lot of towing for my own business, for play, and as a professional truck driver in my past life.
Sounds like you need the right tool for the job. An in town putt putt is one thing, but if your actually doing a real tow, I think your setting yourself up for failure, in a bad way.
The top tow rating is what 5k lbs? Your going to have a tandem trailer with forward position axles to keep the tongue weight in check. Boats fit in that category too from the rear bias. But a transporter? Your going to have a real challenge not blowing past the 500lb mark on tongue weight. Timbrens or air bags can solve your ride height issue, but I doubt the wheel bearings live very long. You may even need an anti sway hitch to tame how unstable it might get. As light at 3rd gens are it’s going to feel like a rodeo with any real speed.
RVs aren’t the same as a flat deck car hauler, not even close, but for my 8-9k lb rv setup i run a 1 ton pickup, air bags, the biggest equalizer hitch you can buy without getting weird about it, and a Hellwig sway bar. Maybe it’s over kill, but it’s comfortable to drive, and extremely in control. I don’t have to play super trucker anymore. Did plenty of that in the past.
I say this as an unrestricted CDL holder and a guy who used to run a 129k permit, I t’s not the moving it that’s the problem. The 4runner isn’t heavy enough for that job, nor is the suspension calibration even in the kinda sorta category for stability. It’s just the wrong tool. 4Runners are awesome, but they have limits.
Keep the 4runner for whatever. Get an appropriate sized vehicle for the towing task. You don’t even have to spend a lot of money. Old 90s and early 2000 suburbans are a dime a dozen for cheap and would do the job your wanting so much better. That’s not even getting into transmissions and if that rear axle can live. Plus the right vehicle will do it safely. The ratings honestly are there for a reason.