Agreed. Every position has a higher success rate in the early rounds. Off the top of my head the SB winning RB's this decade have been Jammal Lewis, Antowain Smith, Forgot-his-first-name Pittman, Antowain Smith, Corey Dillion, Willie Parker, Joeseph Addai....
I realize there are some mediocre QB's in that same time-frame too, but there's a reason why the salaries go QB > LT > DL > RB