is this stock height? and lowest I can go?

[StarDust]

I am wanting to drop my silverado but i don't know if the truck is stock height as other pictures I see stock look a bit lower on the back than mine so I don't know but anyways what is lowest I can go without major rubbing or bottoming out? the rims/tires I have on it right now rub so don't know if going lower is going to make rubbing a major hassle thanks (sorry for the way too oversized image do not know y my phone does that)