shims or plastic spacer thingies? If you are referring to the plastic pieces that help cut down on rust from metal on metal between the springs at their ends... well, they should come with a tab that goes in a hole that keeps them generally in position. If they don't, then, ummm, duct tape? or perhaps drill a hole in there and squirt some RTV or something in to hopefully hold them in place....

91 Jeep Wrangler YJ, carpet, tires, trailer hitch. yea....