...then you have a ton of tire choices. More so than those of us with 16" wheels. 15" is kinda the standard and most tire brands come in 15" size that will fit the X. I think you can fit 31's with either no trimming of the inside mud flaps, or with just slight trimming. Or play it safe and get a set of 30's which do not require any trimming. You will not find anything in a 265/70/15 size because at that size most 15 inch wheels go list their tires in flotation sizes. I'd go with at least 31's if you go off road. Maybe 32's even depending on what terrain you like to wheel on.
If you can read .pdf files (adobe acrobat) go here for a list of BFG tire sizes. Near the bottom you'll see the flotation sizes for 15" wheels. Look for 31x10.5x15