If you look up the stats of Tolbert, you have to throw out 2010 and 2011 and even last year for the Bills as those seasons he was only really a tailback, so at fullback he really topped out at 360 yards a season.
Nall would need injuries and then being too good to be relegated back to being a blocking back ie Hillis. At the moment no team really gives fullbacks anything but scraps for rushing yards.
PS. If a fullback or even a tailback can become touchdown vulture specialists, their careers can thrive and be extended for years if they are on a team without a true bell cow tailback. John Kuhn has lasted into his mid 30's because of this and Matt Asiata hung around the Vikings roster well past his expiry date because of this.