Don't know about Jones

When I look at the stats that Julius put up last year, I see a average of 3.5 yds per carry behind an O line that turned MBIII into a pro bowl back.

I do NOT believe that Shawn had the same luxury.   Neither he (granted his injuries had a lot to do with it) nor Morris could move the ball.  The situation was so bad that Hasselbeck had a great year passing. 

Unless there's an improvement in the O line is Seattle, I can't see Julius Jones being the huge improvement that they need.  It was a rock bottom running program and there isn;t much lower to go, but come on, replacing only the back contends that there was only one thing wrong.

I think you hit it on the head. Aside from injuries, Shaun Alexander was never the same RB after Steve Hutchinson left. While I respect what Alexander accomplished, I do think he benefitted from one of the best O lines in the NFL, and Hutchinson was the key to that.

I'd give Dallas' O line a better grade than Seattle's, so unless Julius Jones can gain yards receiving also in Seattle, he will not be a productive RB. I would not expect to see much, if any, improvement from his 3.5 per carry average.

That being said, I would not be all that happy with having im on my legacy team right now.

I would try to trade him while the expectations are high.  He will end up in the same situation in Seattle as he had in Dallas.  Lots of yards and give way to someone else near  the end zone.