Uh.... It sounds like the rims you put on are alot bigger than what is supposed to be on there. Try putting the 18 x 8s on the back and the stocks back on the front and see if it still rubs. If it doesn't rubs than those back 8.5in rims are too wide for the rear.
Either that or see if it left a rub mark in the wheel well when you pull the rear tire off and see what it is rubbing on. Or see if it marked the tire or the rim and try to line the marks up.
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