The 1.5 body lift does great with as big as 235's on stock rims. The problem you have going bigger is the shock mounts in the back. A suspension lift is the best solution for that. Mine has 30x9.5's on the stock rims and only rubs on extremes, but I don't think 31's would work. (I have 2.5" suspension with the 1.5 body.)
If you want to go to 31's I'll make you a great deal on the black 15x8 Cragars in my pics, IM me. But you will have to do a suspension lift, I was running 31's with them.

2000 KIA Sportage 4x4, Sadly Smushed! Now a 2001 Cheep Jerokee. 3" Rough Country Lift, 255/70/16 General AT2's
http://www.cardomain.com/ride/2292811