It's not the T-case grinding, it is your tranny park "lock teeth".
Usually, when in drive with T-case in gear (not in neutral), stopping the truck stops the trans from rotating, because the tires are not turning. When the T-case is in neutral, the trans keeps spinning because there is no connection to the tires and therefore no resistance. Putting it in park when the T-case is in neutral is essentially like putting it in park when the truck is still rolling.
Obviously, the torque converter spins all the time, but the internals in the tranny don't spin when you are in park or stopped in gear. Well, some do, but in-depth tranny function is a little beyond this discussion, and better left to a more knowledgeable person than me.