You should be able to find that information in your owners manual. I also think you're within your limits HOWEVER if I recall correctly from when I was shopping for a TV there are a few other things you should consider. According to Toyota:
1) 20" wheels will decrease your towing capacity by 500 lbs. over 18" wheels
2) the powered seat option (in seats other than the driver's) decreases your capacity by another 500 lbs.
Since you have the Limited model there is a good chance you have one or both options on your Sequoia. In any case, I'd start by taking your Sequoia to a scale and get an accurate weight.
Payload is the "weak link" in what is otherwise a fine tow vehicle. Our TT is around 6,000 lbs loaded for a trip. Between the tongue weight and our kids (when they were still traveling with us) we were right on the GVWR.