If you have to go Nitto Grappler this size will fit your rim but will likely rub big time since it is 13" wide! LT325/65R18 D 121R
I'd go for the 12" wide, (305s), BFG ATs - or maybe one of the 12.5" wide, (315s), mudders, or Procomp ATs listed here, (because Nittos are supposed to wear quick?).
http://www.titantalk.com/forums/547192-post4.html