running a different size tire

I am running 34" tires on my 99. I still have the 31.50 tire for a spare. Would it ruin anything using the smaller spare for short term. And not in 4-wheel drive?

 

You can do this on short distances and at low speed, without 4x4 mode. Just put the smaller tire on the front, because in case of something going wrong, it's cheaper to replace a hub or a rotor than a differential...

 

A differential is designed to allow 2 wheels to turn at different speeds, it can accommodate different tires sizes. Look at how some auto manufactures include baby doughnut tires as a spare. However, speed should be limited as should distance. Running different tire sizes for extended time will cause excessive wear on the diff. However, if this is on your 84 and you have manual hubs, you could swap to the front with a disengaged hub and minimize the wear.

 

I hope i don't make more left turns than right turns then. Three left turn make a right, dam I'm going to wear out my diff.