Groan! I see both sides of this issue. I totally understand why you and DH are thinking about moving. But I also get why your YDD is so upset about it.
If she were just concerned for herself, I would say she needs to adjust, find more friends in reasonable distance, join activities/organizations that would keep her busy and so forth. But her child is involved and from what you're telling us, your moving may be as dramatic for him as if a parent moved away.
You don't say how you think you'll feel. Are you sure both you and DH will be ok with the distance? Usually, posters come in here complaining that their AC (adult child) and family have moved away and that they miss them and so on. Are you being honest with yourself about how you'll feel? You seem to be worried that you'll be "risking it all" by relocating, so maybe you're not so cool with this idea, yourself?
I can't imagine moving far from my DDs and GC. There's a chance that one or both of their families may move farther away someday. And if they do, DH and I will adjust. But I can't imagine our being the ones to do it, ourselves!
Anyhow, because of your GS, a part of me thinks you shouldn't relocate. But another part of me thinks this shows you and YDD and her family are "too close." And it would do you all good to get some space from each other. In fact, I'm concerned that if you stay, you'll soon be as deeply involved with her other child/children (you didn't say how many she has). Is it possible that one of the reasons you want to move is that the demands on your time are getting to be too much?
There's a lot more for you to think about, IMO, than money here, as important as it may be to "free up some capital" and so forth.