If you have an E350, those didn't get OEM rear sway bars till the 2000's some time. If it's an "E-Super Duty" (original name for what we now know as E450) or any E450, then the Hellwig kit includes hardware to mount the center crosswise part of the bar to the rear axle tubes, one clamp on each side of the differential. You go from nothing to a 1.5" diameter sway bar with poly bushings and will feel the difference right away. The front replacement bar, 1.375" if I recall, is also a marked improvement. That's part because it's so much bigger than OEM, part because it took Ford till 2008 to realize the front bar poked through bushings in the axle, instead of using end links, was a sorry design. The Hellwig bar adds end links.
SOCAL is right about a Ford chassis swaying. Sway produces uncommanded steering. Think of the guy in the back of the old hook'n'ladder truck steering the back when the driver in front didn't want him to...
