My 2017 F350 cc 4x4 gets a severe shaking under down hill braking, get more violent the longer you stay on brakes, this is in tow mode pulling my impact 391 5th wheel toy hauler, has also done it pulling other similar trailers. Doesn't shake under braking at any other time when pulling trailer. Does it on smooth paved downhill roads at 65 mph. Trans downshifts when initial braking applied, shake starts when brakes reapplied to slow vehicle down. Gets so bad have to let off brake and that's not always possible in traffic. No way to duplicate without pulling trailer downhill at speed and braking.