So I messed up. My spare wasn't included in tire rotations and I have about 34,000 miles on the Jeep. Should I get flat and I use my spare, will it screw up anything since the tires will be different sizes?

 

If you get a flat just ride the spare until you can get it patched and then put it back if is that big of concern. It should be good enough to get you home.

 

You've got a bigger problem, at over 34k it's bout time for that new set !

 

AFAIK when you have to use one tire of a different size it is recommended to put it in the rear (non-steering position). It is easier to compensate for steering/braking asymmetry this way, and the differential will compensate for different wheel rotation speeds (but maybe it will heat up a bit - I am not sure).