Plain old black rockcrawler style rims fit on my '02 renegade. They run around 4 1/2" back spacing. The big thing to watch with 15" rims is the clearance on your front break calipers, some rims will not fit over the top of them.
I really doubt you'll be able to run 31.5" tires without a lift. If you can get them on, you'll most likely have severe rubbing.
According to the tire chart:
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?t=958
you are only going to be able to fit tires around 30x10 without rubbing.
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Frankenlift
Boulderbars
All J Tranny skid
31x9.5 SS TSLs!
Detroit Truetrac/Powertrax
http://www.cardomain.com/ride/2062875