A flex plate seldom ever fails. Have you owned the truck since new or is it known if the transmission has ever been out before?
Sometimes when an automatic transmission is removed the installer may make a mistake and not get the torque converter seated on all of the splines.
When the transmission is in place and the trans mounting bolts/torque converter bolts are tightened the flex plate is distorted and under a lot of pressure.
Upon startup the converter may then seat (or not) and the weakened flex plate may crack shortly afterwards or it may go months or years before giving up.
I had a Subaru into the shop one time in which someone had made this mistake and the entire center of the flex plate had peeled clean out followed by the car ceasing to move one inch further.