I recently bought an 02 Sahara. It already had 31-10.5-15's on it. I just noticed that the spare is a 30-9.5-15. Correct me if i am wrong, running 2 different size tire's on a 4wd would be hazardous to it's health!
In a pinch to get to the repair shop or off the highway in 2wd you should be okay. If budget is an issue go to a used tire dealer for a serviceable 31" used tire or I'm sure someone here might have a spare spare. Always best to get the correct sized spare asap though.