Trouble Hooking Up Trailer due to "Gear Slack"????

Same issue here. I find that if I hold my foot on the brake pedal for 3 or 4 seconds or a little more after putting it in park it stops the truck from moving - not 100 percent of the time, but almost. As the above poster stated, maybe holding it in neutral for a few seconds may also work for the OP.

Not sure why this works. Maybe it's because the fluid circulation inside the tranny slows down?