We don't even know if Ricky will get re-instated or when that will happen. It could be tomorrow or in two months. If he is reinstated soon, the Dolphins still own his rights. Do you trade to get him? What if they don't release him? The trade deadline is two weeks. How can you make plans to get rid of Julius Jones when you don't even know if/when Ricky is/will be available?
And I don't think that is even the biggest issue with Williams. He has proven to be undependable. He had talent coming out of college and had a huge year a few seasons ago, but that is not enough to off set the negatives he has brought to himself.
As for Julius, I think at the best Dallas will get what they can get out of him this season and then let him walk. I think he is worth more to the team playing this year than any future draft pick. I doubt he would bring anything higher than a 3rd rounder. He has value as a guy sharing the load along with Barber. It is working for Dallas. Why mess that up?