You're never going to get a RB for a DL. The reason for that is the DL guys are traditionally the last round picks, are picked up and dropped throughout the season depending on byes and matchups, and are just viewed as less valuable in pools. D players are normally plentiful in pools with guys on the wire only having a slight dropoff in value. With O players, the gap widens dramatically.
I'm in a pool with Tillman on my team this year. He's one of my most valueable players point wise, finishing 2nd on my team in pts each of the last 2 weeks. Would I keep him? No. If I really wanted him next year, I'd just beat the D rush a round early and grab him a round or 2 early. There's not a chance I could trade him for a RB unless it was for a low level backup with little upside.
Hope this helps!