Sorry to hear about that. Mickey Thompson are tough tires so i don't know why that one developed those cracks Maybe a manufacturer defect or maybe something the tire caught on the street /trail etc. ? That's also why i buy tire certificates when i buy new, but mostly cause i drive in the dirt on weekends and you never know..., i don't buy certs for any of my cars/suvs that are street only vehicles. So i do that only for the ones i used offroad. I know is not what you want to hear at the moment but maybe something to think about for the next set of 4 or 5.
You're at half life, so you have different options. you could buy 2 new of the same tire and go through their process. but now you have 2 at half life.
or you could buy one used of the same tire from Craiglist/ebay assuming similar wear and assuming you can find it.
Or you could sell the 3 left, and with that money buy 4 used tires from Craiglist/ebay, or buy 4 new tires from anywhere. certs always make sense if youre offroading the truck.
Good luck.