I would go back to the basics, the beginning.
Make sure the trailer has about 12% to 14% tongue weight.
Ensure the WD hitch is correctly set up and you are at 100% FALR. Verify this at the CAT scale.
Consider changing the shocks on the rear of the truck. I like and use Bilstein 5100's.
Do all this and report back. In other words it's helpful to have all the basics in a known configuration, then if there is still a problem, you can start changing things one parameter at a time until you find the problem and solution.
Hope this helps.