Roster Question

12 Team PPR league. Start 2 RB, 2 WR, and 1 RB/WR Flex. I'm currently in 2nd place but leading my division. I'm essentially guaranteed to make the playoffs, so I'm looking to add depth for the playoff run. Here's my roster now:

Chris Johnson (start)
Pierre Thomas (start)
Tim Hightower (sometimes flex)
Mike Bell
Derrick Ward

Andre Johnson (start)
Wes Welker (start)
MSW (sometimes flex)
Devin Hester
Nate Washington

I don't feel comfortable in my RB depth at all. I'm thinking of trading a WR or WR package for a RB. Who do you think I should try and move? Would a Hightower/Hester package get me anything? Or Hester/MSW?

FYI...there's no RB's on waivers worth anything (obviously) and here are the WR's out there now: Maclin, Garcon, Meachem, Avant...