Even though the front wheels may not be under power, chains on the front would help them, and the van, go in the direction you're steering. Not as good as chains on 4 driving tires, but better than on just two tires.
Interesting about the not not to use chains on the '08 E250. Last I knew SMBw sells chains. Personally, I prefer the Rud chains that Bill Burke sells (even though I don't have them).
SMB-less as of 02/04/2012. Our savings account is richer, but our adventures are poorer.