Bucking when slowing to a stop with loaded trailer

[Diamondisfat]

My 2018 Ram 3500 Diesel is in the shop. They can’t figure out why, when I tow a load, that the trailer & truck rock back & forth at a gradual stop.
It’s never done this before. It has 16,800 miles on it. They say it’s nothing to do with the brakes.
It doesn’t happen when I tow gooseneck or dump trailer empty. Only when loaded does it jerk & rock back & forth at a slow stop.
I’ve unplugged the trailers to see if it’s an issue with trailer brakes but it’s not.
Thank you for the help!

 

[Travelin Ram]

What happens if you put the transmission in neutral while braking?

 

[Diamondisfat]

Didn’t try that. I will though. Thank you!