I have read that a 31 is the biggest tire you can fit in the stock location. Is there any modification needed to wiggle a tire this size into the back? I've been thinking about bumping up the tire size but didn't know how big of a hassle it would be to fit a 31 in the back.

It would be very tight, maybe invest in a rear bumper with a tire rack swinging hitch

I thought I read about someone putting a 31 in the original spot but I can't find the thread. I'd rather keep the spare in the cargo area if I can. Anyone?

I had a 31 in my original spare location. had to use a longer bolt to hold it in place but it fit tightly.

I've had 31 inch tires that measured anywhere from barely over 29 inches, to just under 33 inches tall, and they were all plainly marked as being 31X10.50-15. It depends on which brand you have. My opinion is that the less expensive tires are generally smaller than the more costly brands. Your tire might slide in the original mount with no problem, but again, it may not. Break out your tape measure and check it out.
I have a Michelin A/T 31X10.50-15 as my spare, and it will not fit in the original mount area. It just barely fits between the back of my rear seat and the tailgate.

slap it on the roof! thats the ticket!

^ that's what I did. It makes her a thirsty jeep though. I'm planning on rear mounting when this new bumper is finished. I'm selling the old new one. (check out the thread in the forsale section... Oneoff rear bumper)

I removed the metal bracket the origional spare sat on and ran the origional bolt through a lug hole using the factory wingnut to secure it. My "31 spare fits in the stock location this way.

my stock rubicon/mtr fits in there with the metal bracket. have to put the good side of the rim towards window and put the screw through the top lug hole... fits but very tight.

I have fit a 32 in the back very tight but fits.

1. I extended the bolt by cutting and welding in a 2" pc
2. Then take the plate off that is mounted to the floor.
3. Then hammer the wheel well down a little more then it is.
4. This was not needed for the 31 but will have to to fit the 32 I removed the top bracket for the back seat (just the side the tire is on) and extended the hole so that the seat would sit forward a bit. Very easy to do and works great