If a woman wears high heels, is that considered immodest (according to halachic jewish standards)--assuming everything else she's wearing is within the guidelines of the halachot of tzniut?
As alluded to in the comments, when it comes to matters of tzniut there are often varying community standards. That said, there is an interesting source that seems to be discussing high heels (or at least an ancient version of high heels). In Isaiah 3:16 the prophet states:
Moreover the LORD said: Because the daughters of Zion are haughty, and walk with stretched-forth necks and wanton eyes, walking and mincing as they go, and making a tinkling with their feet;
Commenting on the words "the daughters of Zion are haughty", R. Isaac Abarbanel writes:
שהיו בעלות גאוה או שהיו מגביהות גופן למעלה ע"י נעלים גדולים
That they were haughty, or that they would raise their bodies higher by means of big shoes.
Abarbanel is apparently noting that the Hebrew phrase גבהו בנות ישראל in the verse is somewhat ambiguous. While it could mean that they raised themselves up with haughtiness, it can also mean that they physically raised themselves up by wearing shoes that made them look higher/taller. If this is the meaning of the verse, then this is what the prophet was strongly criticizing.