On the above mentioned trucks with more than one cracked flex plate in several years time; I'd have to question the crank shaft thrust bearing. If it is worn beyond limits it could be allowing the flex plate to walk back and forth under various loads; causing undo lateral stresses on the flex plate. On a manual transmission truck you can determine worn crank shaft thrust bearings by watching the crank pulley at idle while somebody presses the clutch in and out. The crank pulley will walk fore and aft as far as the thrust bearing will allow it to.

On an automatic tranny, I do not know of any way to check the crank shaft thrust bearings, short of dropping the oil pan and dropping the thrust bearing for measurement.