Help me fix my wobbly windows!

OK so this has been something I have been ignoring since I got my '70 duster six years ago. When the door windows are anywhere in between fully up or down they wobble back and forth and squeak like crazy as I drive down the road. Looking at the window mechanisms it seems like there's supposed to be an "insert" that holds the window glass tightly to the vertical tracks. I have no idea the technical name or where to get it. What do I need to do to fix this annoying problem?