-18 offset should be plenty enough to clear 12.50's without rubbing. If you stick with a 9" rim you are basically moving the rim and tire out from under the truck about 1.4". That combined with the 12.50" wide tire will definitely make it stick out. As for the backspacing....it's basically a product of the offset and width. If you already know the offset and width that you want, then you don't need to worry about the backspacing. If you stick with a -18 offset then the only way to change the backspacing is to change the rim width. Make sense? Hope this helps.
06-08 chrome clads w/33" Cooper Discoverer H/T Plus, Mopar tailgate spoiler, debadged door trim & tailgate emblems, 13" antenna, LED dome & maps, Magnaflow 22" SI/DO, 18% tint, "HEMI" hood spears, VHT tinted tails & third brake light, Lamin-x optic blue fogs, leveled w/McGaughys 2" drop shackles.