Off the top of my head the most aggressive tread tire I can think of in 18" would be the BFG AT KO's in size 265/65-18. I also run that same size on my GX with T4R Ltd rims but I have Bridgestone Duelers instead of BFG KO's. That size gives you about an extra half inch of ground clearance vs. the stock 265/60-18 size.
For true mud tires you will probably need to drop down an inch to 17's