I had the same problem once.
you need to get yoru front end done.
Or better yet...
Jack your van up just so that the front 2 wheels are off the ground an go to them an wiggle them from side to side if they move then you need to go get your front end tightened or re-built or replace.
I needed mine to be replaced.
sorry for your bad luck dude.I have econoline 2004 350, how hard is it to change front shocks, I have speed woble.?
Need more info but I would think its a balance or tire problem.
