Spare tire & trac loc rear

I have a trac loc diff w/ 350 gears (if it matters) and run 295/50R15 tires (26.7" dia.) in the rear and 235/60R15s (26.1" dia.) in front. For space reasons I have had a late model temporary spare tire in my trunk that reads 125/90R15 and measure approx. 23.5" in dia.

It resently occured to me that using the spare I have would bind my rear end due to the difference in tire dia. and the trac loc. Is that correct, and if so how close in diam. should my spare be to either front or rear tire dia to use in either place without doing harm?