Boulevard

It sounds like the problem involves the whole truck and not just one part and to me it sounds like the trailer sway program is kicking in or maybe the traction control program. These programs will apply the brakes, retard engine power and slow the truck in an attempt to stop what it sees as a trailer that is mowing to much or wheels that are spinning out of sync. It may not show a code because the truck is doing its job or you may have a sensor of the verge of failure but not yet fully gone.