NFL Connections #93, Answers
The full solution: all four groups and the 16 names, sorted from the easiest group to the trickiest.
2000s workhorse backs
Edgerrin James, Clinton Portis, Corey Dillon, Rudi Johnson
1990s power backs
Jerome Bettis, Terrell Davis, Curtis Martin, Eddie George
Speed backs
Chris Johnson, Jamaal Charles, Reggie Bush, Darren Sproles
Hall of Fame backs (classic)
Jim Brown, Gale Sayers, O.J. Simpson, Earl Campbell