SUV sway when towing? not trailer sway - SUV sway

Tow vehicle is a 2009 Ford Expedition EB 4 wheel drive 5.4 with factory tow package and "tow- haul" selector switch- this is 4th one we have had going back into late '90s.
These are rated to pull better than 8000 lbs with tongue weight of 600 lbs max.
Boat is a 235 Mako with total towed weight of 3300 lbs plus trailer - trailer dual axle aluminum. Tongue weight is right at 600 lbs.
I have pulled this boat with 2004 and 2003 Expeditions with no issues.
The 2009 has a sway in the SUV (not the boat) with tow/haul turned off - it reduces with tow haul turned on but is still there. Tires are LT with 45 psi.
Any ideas on cause of sway? <!-- / message -->

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Slide the boat back and try less tongue weight.

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Originally Posted by FMP

Slide the boat back and try less tongue weight.

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Agreed, you only need about 400-450 lbs on the tongue. I bet the truck drops close to 3” when you hook up, that’s taking too much weight off the front. I’m pretty sure the ‘09 has a different rear suspension with softer springs than the other 2 you mentioned.

Check all the rear independent arm, knuckle and link bushings and joints for play while you're at it. Also make sure your rear alignment is on spec.

Doesn't tow/haul just lock out overdrive? I think Ford manual says tongue weight should be 10% of towed weight, too.

What brand of LT tires. I once bought some house brand LT 10 ply tires from Costco for my 3/4 ton suburban and they swayed more than P rated 6 ply tires even with 70 psi in them. Took them back and bought name brand tires and no issues after that. Here is a good tire test to test the sidewall strength of your tires. Stand at the rear of the parked vehicle (without anything attached) by putting your hands of the rear fender just in front of the tail lights and start pushing the vehicle side to side. I was so surprised with cheap quality side wall tires the truck would sway a good 1-2" side to side when you kept pushing on it. The whole vehicle will start to wobble around. Then try it on say a dually pickup for comparison that has virtually no side to side sway because you have 4 tires at the rear to eliminate any side to side sway. You have to find a tire with really good sidewall strength. They can say 6,8, or 10 ply but they don't say how thick the plys are and how rigid they are.
I also think 600 pound tongue weight is too much for your vehicle. 400 pound sounds right - especially if your vehicle is loaded down with cargo and passengers when towing.
I hope you find your issue. It really sucks going downhill @ 65 mph when a transport truck goes sailing passes somebody and you encounter the trailer sway and think you are going to loose it.

aren't those models known to have faulty steering dampeners that cause that kind of issue?

"especially if your vehicle is loaded down with cargo and passengers when towing"
Oh yeah - SUV load to the max when we take boat to FLA for the season.

"aren't those models known to have faulty steering dampeners that cause that kind of issue?"
This is first I have heard of this-

I would try reducing the tongue weight. There isn't a steering damper in your truck, Mr Burns must be thinking about the 250s and 350s. V-bottom also brings up a good point, tire construction can make a difference. Just because it's rated to handle a specific load doesn't mean it will drive well when loaded close to it's rating. It's a static rating. Also, when you say it's reduced when in tow/haul, that's likely because there is little to no "float" time, the drivetrain is always loaded, whether it's pulling or engine braking. Over the years the pickup (F-150) springs were softened to improve ride quality for the snowflakes that pack groceries in the bed once a month, but I don't think they changed the irs Expeditions. I could confirm that with a vin number if you like. Either way, I'd try reducing your tongue weight.

I love those load leveler hitches, put more weight on front tires....... Sway almost always occur from too little weight on front tires, the rest of the time from wrong or cheap tires, bad front end parts or weather/road conditions. My suburban tows better on a quarter tank of gas or less regardless of tongue weight. Air shocks or air bags in the rear might help