Ok .... the 235/75/15 is just under 29 inches. That is 2 inches less than the 31 inch tire. since the axle is in the middle of the wheel, you only get half of that 2 inches, so only 1inch extra clearence. That is for the back diff, the front diff will be higher because it is an independant axle, and the suspension lift bends the wheels down away from the body. The do-it-yourself lift is a 5 inch lift meaning the 3 inch body lift plus the 2 inch suspension lift equals 5inches. Add the 31inch tires, the body will be 6 inches higher off the ground. This means there is more room for a bigger tire, and more room for that tire to travel. Since the body is higher, your less likely to drag it on rocks and the like, and you can go through deeper water without sucking it in. You just wont get that rear pumpkin up any higher unless the tires are taller. Make sense? The 235's are about 1.8 inches taller than the stock 205's. What I liked at first was that I did not need to use ramps to change the oil like the cars I have had. You could do the gas tank lift too, cause every little bit helps.
Hope this helps,
Jeff