On July 18, 1984, catcher Darrell Porter belted a two out walk off grand slam that gave the Cardinals a 8-4 victory over the visiting San Francisco Giants. Porter hit 188 home runs in his career, with the first coming in 1971 as a member of the Milwaukee Brewers. He spent six years in Milwaukee, and four years with the interstate rival Kansas City Royals, before signing with the Cardinals in December of 1980. In his five years in a Cardinals uniform he put 54 over the fence, which included his only two walk off blasts. The one that came on this day in '84 was the second walk off. The first came on May 20th of that same season, which came in the form of a two run bomb in the ninth inning that led the club to a dramatic 3-2 victory. The MVP of the 1982 World Series parted ways with the club following the '85 season, and signed with the Texas Rangers where he spent the last two years of his career.
Check out the box score here: http://www.baseball-reference.com/boxes/SLN/SLN198407180.shtml
I actually use d Porter's first walk off blast on the page earlier this year. You can read about it here: http://tinyurl.com/PorterWalkoffMay84