I would not trade Best for Jackson at this point. Both are banged up, but I think Best is just fine and has more upside than Jackson actually. I would not trade Johnson because you're just hurting your WRs for RB depth.
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