My backyard was pretty much entirely paver stones and I'm pulling them out to put in grass. I've pulled up many of the stones and underneath them is sand (fortunately they did that right). My question is, do I need to get rid of the sand before planting grass (presumably) or can I just till the sand into the dirt underneath it?
I would check the base under the sand. Did they put in crushed stone for drainage? If it is just a few inches of sand over existing soil, then I would add compost, and then till it all together. The organic matter in the compost will greatly improve the soil.
If there is a stone base under the sand, then you may need to take it out.
Depends on your ground/soil type but lawns generally like well drained soil so having sand mixed into it would be good.
Unless your soil is very nutrient poor, thin sandy soil already
It depends on the depth of the sand and the type of grass you intend to plant. There are many grass varieties that do very well in sandy soil, so you should have no problems leaving it in place and either mixing it with the topsoil that is presumably still beneath the sand, or getting a small amount of soil in to mix with it.