The last of the 121 times Michael Sheridan saw the Grateful Dead was in 1995, at a blissful concert on the San Francisco Bay two months before Jerry Garcia died. After that fatal blow to the group, Mr. Sheridan moved on, too. He avoided other jam bands, even ones featuring former members of the Dead. None could live up to the original, he thought.

Last January, however, when the surviving members of the Grateful Dead announced a reunion to mark the band’s 50th anniversary, with three concerts in Chicago during Fourth of July...