Early 70's GMC's had the upright tail lights. Maybe the truck in the pic is not this old, but from the grainy pic this was the first style that came to mind, especially with the big box.
As an aside, gut wrenching.
Also, I know these "living estate" sales are more popular these days, but opening your doors to hundreds of complete strangers into the house you're still living in, is just not a great idea. You're bound to get some unstable people or shady people looking for crimes of opportunity, as some of the items at these things are of decent quality and price, not just garage sale junk.
Show up with 20 minutes left in the all weekend sale, case out what hasn't been sold and show up the next day thinking someone is not home..or, not caring if someone is). Depending on how messed up said person may be about the situation, things escalate quickly and with a little kid caught in the middle, the grandparents do whatever they need to to protect the child (maybe even getting hurt in the process). Making some assumptions here, but hopefully sorted out soon.