Alan,

Check your rear wheels with the van jacked up.  Release the parking brake and spin the wheels.  Listen for
grinding, etc.

Try to move each wheel with your hands at the 9 & 3 and 12 & 6 O'Clock positions to check for play.

Did you notice the problem after the new tires were mounted, or was it there before?

Finally, if you have aluminum rims, they may have bent them while mounting.  This is difficult to diagnose
unless the tire is off the rim and you can roll the rim on a flat surface.

HTH,

John Haddad