First thing I'd suspect is sway bar bushings as Boomerang mentioned, as turning the van loads the van's weight laterally (body roll), of which the sway bar compensates for by equalizing the load on the suspension.
You can test this out by pushing the front bumper up and down at the corners and getting at least three inches of bounce travel either way. If the squeaking starts, then spray some soapy water into the sway bar bushings and bounce up and down again. If it still squeaks then spray the end links next. If the squeaking stops then install new sway bar and end link bushings.
Just a heads up, If you opt for longer lasting polyurethane bushings, then make sure you lube the bushings with the proper silicone based synthetic waterproof grease like the stuff that prothane and energy suspension uses in their installation kits.