2012 Skeeter 20i - Trailer question

I posted this on another forum and didn't really come to any conclusion, so thought I'd try here.

Puzzle for you guys.

I have a 2012 20i, bought new.

I had a 2013 Ford F150, when I'd pull the boat/trailer between 67 and 72 MPH, the truck would shimmy/shake noticeably. Not crazy, but definitely noticeable to the driver and passenger.

I had the trailer checked at a Skeeter dealer. Nothing found. I had the truck checked at a ford dealer. Nothing found. I've replaced all the tires over the last several years. Still does it.

I pulled the boat with a buddy's truck, no noticeable shimmy. Skeeter dealer pulled the boat with another truck, I believe an F150, no noticeable shimmy.

I bought a new 2018 F150. AND still between 67 and 72 the shimmy is felt. It's definitely noticeable. Right at about 67 MPH the vibration starts and stops at about 72 MPH.

Any ideas what is going on?

Balance the trailer tires. noboby seems todo this. I don't know why.

Thanks, will give it a try.