The rubber insulator is a thin piece sandwiched between the two halves that comprise the pulley. There's the actual pulley center, then the insulator, then the belt drive part. It's hard to spot without scrutinizing some. Also, did you try re-seating, and properly torquing (ie not using an impact) to see if it still wobbles? I've seen them run true when re-torqued after being wobbly for a while.