I don't see a R18 for the MT/R - http://www.tirerack.com/tires/Spec.jsp?make=Goodyear&model=Wrangler+MT%2FR

I had MT/Rs on my X and loved them. They are a very heavy tire and you will loose MPG and power as compared to a BFG A/T.

Good luck on finding the right tire for your 18" rim. Its a nice rim!!!