When I put them on mine at work I had to choose the plus zero sizing I believe. If you want to stay stock size you'll have to go with either the LTX M/S2, a great tire but not as good off the concrete jungle or the Goodyear SR-A another good tire same downfall as the M/S2 in that it's off road behavior is substandard for an "A/T".

285/55R20 would be close in size, about a 1/2 inch taller. Note that the 18" tires for some reason are 32.1 and the 20"s are just a hair under 32". Why I have no idea.

The LTX M/S2 is a great tire and will take care of you till somewhere between 80 and 100K miles. They last longer than the AT2 hands down but are not as aggressive.