It's not a bad trade, per se, but I don't think it accomplishes what you want to do (make a playoff push). Since you only start 2RB and no flex, Johnson/Ringer would just sit on your bench unless or until something happened to Rice or DMC. Unless you believe/fear DMC's injury is long-term, there's no great benefit to the move. For this week, Tolbert is a fine fill-in for DMC.