I'm not sure you can afford getting rid of Forte with your RBs. I think Marshall is just fine and the difference between Marshall and Johnson is not as big as Forte and your 3rd RB, IMO.
12 Team League. We start 2 QBs and get .5 PPR, otherwise standard scoring
QBs: Stafford, Fitzpatrick, Collins
RBs: Charles, Mendenhall, Best, Starks, Carter, Karim
WRs: D. Jackson, Marshall, S. Smith, Green,
TE: Hernandez, Cook