Just a thought,
I am wondering if you are towing anything? Also, if you are doing burn outs and the back end is twisting out from behind you, that is putting the whole load of the truck on those rear plates. I have a block kit on my truck that they say help, I cracked my OEM ones. That is my suggestion on the blocks, but I have never heard of somene snapping leafs before that is crazy. Could be anything, combination of mudding, burnouts, curbs, pot holes, daily driving stuff, or what is very unlikely but in my research of your awesome mysterious problem, I have read that if they torque the bolts to tight on the plates for the leafs in the rear they can actually crimp the metal on the leaf and create a weak spot, or a great point for a break/tear of the leaf. Hope this was of some help.
Brad