jamas, do you have any photos of the cracking? generally if i only see cracking around the shoulder of the tire, i dont recommend new tires unless the cracks are large enough to fit the edge of a coin into
when you see cracking in between the treads of the tires or right around the rim, thats when i start to get concerned
i think UV must have at least a small part of it, ive never seen a spare tire mounted under a vehicle with dry rot on it
I will take some pictures this evening. The tread looks great, just the shoulder has the cracks. I will take a picture of the worst of it and if you say I am good, I will trust your judgment and hang onto these tires a bit longer.