Same transmission problem happened to me little over a week ago, It was my pump. Try checking the ATF level with the truck not running, mark it on the dip stick then start the truck and check the level again. You can do that or check the port on the side of it for psi output. My moneys on the pump but may be just maybe the filter dislodged its self from the pump inlet and the pump isn’t getting any ATF due to lack of vacuum.