you can't put it in the stock place, my 30x9.50 wouldn't fit so i put on the roof rack. a 33x12.50 would be tight fit laying in the back also could hit you in the head if you had a rollover or crash
I have a 32x11.5 ratchet strapped down in the back, laying flat. It might be a pretty tight fit for a 33, but maybe... Definately tie it down securely so it doesn't become a missle in an accident or rollover.
The biggest tire that will fit in the back standing up in the stock position is usually a 30x9.50R15, although I have seen a few XJs that have trouble putting a 235/75R15 back there. Anything larger has to be laid down.