Whisk the grape jelly and barbecue sauce together in the crockpot insert until mostly smooth (a few jelly flecks are fine).

Add the frozen meatballs and stir gently to coat every meatball in sauce.

Cover and cook on low for 4 hours, stirring once at the halfway point so the bottom layer doesn't stick.

Check doneness: the meatballs should be heated through to the center and the sauce should coat the back of a spoon in a thin, glossy layer.

Switch the crockpot to the warm setting and serve directly from it with toothpicks, stirring occasionally so the sauce doesn't separate.