With plans to go to 33's, 4.88's would be the correct size, but with the possibility of thinking you may run 35's, you should go with 5.29's if you are going to buy new gears, if trying to do it as frugally as possible, as H8PVMNT mentioned there are oem 4.88's available, there are also oem 4.56's that would be the correct set for the 31's you have now, and they work well enough with 32's if you wanted something in between 31's and 33's.
5.29's will help improve your city/rural road mpg, but likely loose some on the HWY w/the rpm's being higher, might find a happy medium between performance and hwy mpgs with 34's over 35's.
