I should point out that you can secure your spreader firmly with a two inch piece of chain bolted to the left rear corner of the bed. Use a sturdy dog snap. Best if you replace one of the handle bolts with a stainless steel eyebolt. Snap it on, raise the tailgate... and the spread cannot move left, right or tip over. Add padlock if the truck will be unattended, for extra security.
Click my name for pictures--but they do not show the set up close enough.