Foster for sure, Oakland's run d is terrible. Id then go with Gore, he's among the league leaders in targets, so he's gonna get you points on the ground and in the air.
Also, you absolutely don't need all those backs, if you cant move McFadden, and you're worried about Gore.. try moving Gore, you have plenty of stud backs. McFadden and S. Jax, are IMO, your best trade bait, I cant imagine there arent some really RB poor teams that would make some moves for one of those guys. I'd keep a hold of Gore, and Foster for sure.