It really depends. How much of the rubicon do you want to do? There are plenty of trails in the park, and all are varying difficulty. I'd suggest looking at maps, they are usualy lined out with difficulty levels. Some parts can be handled easily by mere stock vehicles, even (gag) heeps, and then there are parts that can only be done by rock crawling buggies. A set of 33's would make life easier, but 31's can handle a fair chunk of the place. Mainly, how good are you at driving her? 31's or 33's don't mean squat if you can't figure out approach, how far you can lean her, etc, etc, etc. I'm not trying to scare you off, as it is a fun place from everything I've heard, but as in all wheelin trips, the difference between a good one and a horrible experience depends on your preperation.
M.M.