A good bit depends on what type of wheels you will be putting the tires on. If you are buying alloy wheels you can go with a 31x10.5 easy, 32x11.5 might be pushing it weight wise. If you are going with steel wheels would be better to stick to 30x9.5 probably. I bought my red truck with 235's on Steelies and swapped over 31x10.5 on alloys and performance/mileage is the same if not maybe a tiny bit better. The weight in running a steel wheel is a big factor. I know you haven't decided what tire your going with but I will use the BFG Muds as an example:

235/75 - 37lbs
30x9.5 - 40lbs
31x10.5- 43lbs

Presuming your running 235's or close to that, going up to 30x9.5 your adding 3lbs a tire and going up to 31x10.5 your adding 6lbs a tire. I have never weighed a Mitsu Steel Rim, vs the Mitsu Alloy rim but I am taking a guess and saying the alloys are at least 3lbs lighter than the steels. That would make running a 30x9.5 on steel about the same as a 31x10.5 on an alloy. This is all under the pretense that you don't want to loose any power/torque. If you can live with a little loss 32"s aren't a bad option, and 33"s aren't horrible but you may have some issues in mountian/hilly areas with power loss.

I mention this because you don't want to get bigger tire fever after buying the 30"s. Its true if you buy 31"s you will want to get 33"s (I had heard this but found out for myself its true). I can only imagine what it would be like going smaller even with a set of 30x9.5.