With the latest patch it needs to be moved to a very particular spot it seems. The "too heavy to move" notice should only be showing once you've dragged the rock too far from it's original placement, before that it should say "No room to place" or something. While it still says "No room to place" you should be able to move it as long as you can find that one spot it can be moved too.
(The reason it says "too heavy to move" when you've dragged it too far, I think, is because Strength determines how far you can move objects. But with a strength of 17 the "sweet spot" should be well within that reach)
