I bought the DeadSkin at a car stereo place. It's surprisingly expensive, obviously a lucrative profit item for the companies that sell it. Some folks have used some sort of gutter or roof insulating mastic that they bought in sheets at Home Depot but I never confirmed if the local one had it. For soundproofing, you need a material with mass so that it will reduce vibration modes. That's why the soundproofing sheets I have seen are tar based with a foil covering. Light material like fiberglass fuzz won't do anything to dampen metal ringing.

By the way, one needs to convince one's spouse to not slam the tailgates vigorously, just lower carefully.