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Toyota brought the Sequoia into the US market as a competitor to the Tahoe (and other full size SUVs). It's just supposed to be an affordable/bigger Land Cruiser for the American market. Towing was not a priority in their design. That being said, my money would be on the transmission being the weak link. Perhaps you can find some data to support that. I know they've been using that same basic Aisin transmission for many models since the early 90s and I'm sure it wasn't designed for HD towing.

 

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I now tow a heavier boat with the Sequoia. I think it's over 5k lbs fully loaded. I've done a couple of things that make a big difference when towing at altitude. Now I've done this with and w/out so I know it made a big difference.

The 1st thing I did was to use premium at altitude. Made a big difference. One year going up a grade I was doing 35-40. Flashers on and going as slow as the fully loaded semi's. This was in UT on I-80. Next year used premium and was doing closer to 50. I was shocked.

Second thing I did was replaced the fuel injectors with BA Muffler's injectors. More holes/injector than stock. That also made a huge difference. Burned more fuel but......it allowed the motor to run better and to not to constantly downshift at every little incline. So much so that I cut at least a couple of hours off the drive time. Even my daughter noted the time difference. She noticed that we were filling up in Cedar City while it was still dark and
in our previous trips it was always light.

As for power with the injectors? I was passing semi's doing 80+ at altitude. Something I could NEVER do with the old injectors. Granted, I didn't maintain that speed for very long but it did give the ability to do it. That was impressive IMO.

I've driven that I-15 stretch when it's been 110+ outside on more than a couple of occasions. Never once did the temp gauge move. Overheating has never been a concern of mine.

If the Sequoia was a 4x4 I'd probably keep it until I die. Plan to move from So Ca so on the fence about whether or not to keep it.

Where did you buy those injectors? And were they intended for a Tundra originally?

Given the current prices on these things I'd buy a 4x4 of the same year and switch over all of your upgraded parts. You'll still be able to get good money for your 2wd.