I'm going to throw some stuff against the wall and see if it sticks...
Is the sway control on the truck turned 'on' or 'off'? Change is and see if it makes a difference. I remember there being the option to turn on/off while the truck is parked.
Location of the water tanks. Try filling one that is in front of the trailer axles. For example, my fresh is directly above the axles, then my black, then my grey being closest to the tongue. Add water to the appropriate tank to increase the tongue weight.
Are you returning the proper weight to the front axle of the truck, and not too much (or not enough)? I had the E4 on my trailer when I towed with a '97 GMC K1500. It was rock solid (handled slightly better than with my 3/4-ton now without WDH).
Truck and trailer tires filled to max sidewall pressure.