I saw somewhere (maybe around here) that with a Ford E series van, steering issues are something like 20% about toe-in and 80% about caster.
Your 2006 is really easy to add caster to. It's just a big offset bushing that drops right in. I bet pretty much any alignment shop that does trucks could take care of you.
On my 1983 you have to remove the radius arm, drill a hole, fit an adjustment plate and re-assemble. BLAH! At least the plates are cheap. I got a set for $14 off of ebay.
Hope it all works out for you quick and easy.
Jose