There have been lots of thoughts and suggestions on this. The criteria posted by the OP gives me some red flags:
> 35" tires with 45-50 psi? That's a non starter for me. What is the rim size? What is the width? If your tires are 35/12.50 like I think I read you have to have wheels that are no less than 8-9", preferably 10 or your sidewalls are going to flex an inch or two to the side every time that trailer pushes them. You should have LT tires that would run about 80 psi; not 45-50. With that low of a pressure on tall tires you can't help but sway.
>Air bags at 8 - 15 psi. What brand of air bag? What are its specs? I just traded my 2012 pickup with AirLift 1000 air bags. Recommended pressure for towing was 35psi.
>I see a WD hitch listed. Nothing about any kind of sway bar or sway control. I would recommend a hitch with built in sway control. I have an equalizer and love it. Either way you have to have some sort of supplemental sway control with 35" tires.
>Your tongue weight is already 1300 lbs. You don't need any more. What kind of receiver do you have on your truck? What kind of weight will it support? 1300 lbs is about max for a 9000 lb trailer and I don't know that you need to be at the max. I do know you need a HD receiver when you get to those kinds of hitch weight.
These are simple things and things that appear to me to be obvious reasons for sway. With the numbers you posted I would not think about taking off in your truck pulling a 9000+ lb. trailer.
Fix the things I listed and I think you will see a significant reduction in sway and then you can move on to fine tune things.
