A Shadowdancer isn't simply hiding: she's using a Supernatural ability to not be seen while not having anything to hide behind – she's not actually in the shadow, she remains in plain sight but unseen. True Seeing will work on her.
A Ranger is simply hiding. His Extraordinary ability allows him to disappear into natural terrain while being observed, but it's a mundane hiding-in-the-grass he's doing. True Seeing won't work on him.
In a fictional sense, this should be sufficient. The Shadowdancer is using magic to conceal herself, while the ranger is using mundane skill. True Seeing penetrates magic to see things as they actually are and will show that the Shadowdancer is actually standing in plain sight in a shadow that has been magically moved/altered/detached.
Rules-wise, a bit more work is probably necessary. The description of True Seeing doesn't list anything that seems to cover this case, and an easy answer is simply that if it doesn't say it covers it, then it doesn't apply. However, this is a simple proof that it applies to the Shadowdancer:
- True Seeing "confer[s] on the subject the ability to see all things as they actually are."
- The two lists of things True Seeing are divided into magical and non-magical effects. The magical list is things it works on, the non-magical is things it doesn't work on.
- "Supernatural abilities are magical"
- True Seeing allows the subject to "sees the true form of polymorphed, changed, or transmuted things"
- There is no way to interpret a Shadowdancer's Hide In Plain Sight without it being a magical change.
- From all of the above, True Seeing should see a Shadowdancer using HIPS.