If you absolutly have to, you could put it on to replace a front wheel. I say a front wheel because when you are not in 4wd it is an open differential so the wheels can spin at different speeds. I still however would not recomend doing this for more than LIMPING to the nearest service station.
If you got a flat in the rear, especially if it has a limited slip diff, or even without, swap the spare with one of your front tires, and put the tire from the front on the rear axel. This would also apply for people with lockers.
I would deffinatly look into getting an appropriately sized spare, or if you can't get a new tire/wheel, then just find a tire that has the same DIAMETER, so if could be used if needed.
Just playing around with a size compairison, if you had 305/60R18 tires, and your spare was still a 16" rim, you could put a 245/85R16 on it and have a nearly identical diameter. that would be if you could find one. But I would recomend getting a matching wheel and any similar sized tire. I have a stock jeep spare tire and after market tires on my jeep. It looks funny, but it gets the job done.