If I were you I would just keep Slaton, and be happy with 1800 yds and close to 10 TDs.
IMH Slaton > Ward. We have seen what Slaton can do as the primary ball carrier. He may lose a few TDs, but that only happens if the Texans bring in a big back, and there is no guarantee that they do. I also don't have Slaton ranked much lower than SJax.
I would only go after SJax, and then only if the price is right. If he will do Boldin straight up for SJax, that would be the best case for you. However, I think he is going to want a pick, and I wouldn't even throw in more than a mid round pick(maybe 6th round).
12 Team 1/2 PPR Dynasty Auction League.
QB: Cutler, Hill
RB: Turner,Slaton,Benson,Mendenhall,Harrison,Robinson, Greene
WR: CJ2, Fitz, Smith(NYG), Brunce, Morgan,Bryant