Yeah, we've pretty much covered all the different things so far.
Here's a few more answers to questions if it helps...
1- No, truck is smooth, only happens when towing.
2- No sway whatsoever- straight!
3- Trailer is level and trailer ball is perfect height. Set by NJ cert. trailer inspector friend.
4- 2 place Aluminum Worthington open trailer. Single axle.
5- TRUCK tires in really good shape, some very light feathering took place (outer edges-front) , and are now on the back. I suspect this isnt helping... but within a few more miles, it'll fix itself mostly. The vibration did not increase or decrease, AND the MPH is identical.
NONE of this is felt when truck is not towing (empty), or even if trailer is attached but not loaded with bikes.
I am indeed swapping trailers with a buddy soon as a final test.
My front end doesnt budge, but I'll measure it to see. My trailer is all of 550 lbs unloaded, and add my 700lb Quad, or my 700lber plus my cousins 550lber, it doesnt make a difference, so long as the trailer has weight on it, it vibrates through the steering wheel pretty steady.
Airbags.. My trailer and truck are very well matched right now, even with 2 bikes the truck doesnt even sag an inch (again, visually speaking, havent measured, but will soon.
There. Ignore Xterra pic is old truck/trailer.
The trailer looks a little high, but its more the angle of the trailer, it's actually quite level.
I wonder if my seven yer old in the middle is the cause of all this?