Is it really?
No, it is probably not — not even in the Peter Jackson version of Arda.
- Why was such a thing said at all?
Peter: The line at the end of this scene was an ad-lib of John’s, when he says, “I always like going down south, it feels like going downhill,” which was just something he threw into the end of the recording session that we did; but he often comes up with those really nice little lines that… We can always try to find places for them in the film.
The Two Towers Extended Edition Commentary Disc 2 - Transcript
To put it another way:
An ad-lib was made by John Rhys–Davies at some time while he was recording lines written in the script. Then, during editing of the movie — when raw video and audio are trimmed, sequenced, combined, and post–processed, — the editors and producers were looking for somewhere to insert that particular line. Alas, however, we don't know how much time they spent deciding that, nor their exact considerations made during that decision.
So, we need to make an educated guess as to what happened.
- What could've been John Rhys–Davies' inspiration or motivation for inventing the line?
Well, we lack much of the situational information which could be gained if we were either present at the aforementioned session or had access to the unedited audio recorded thereat.
I can best figure that it was a technique actors will use to help themselves embody a character. I never learned a proper name for it — probably a Method thing, for those who are curious, — but it consists of saying lines which were never written but which emerge from your understanding of a character. We could call it ‘emulation’. If it is indeed Method, then it involves an actor inventing character from means other than script analysis or the like. Voice actors do it occasionally during warm–ups. It has other uses too, but that's enough about that.
Of course, John could've said the whole thing in jest or in a moment of silliness.
Either way, it seems that he was attempting to emulate lines like these:
But if I had seen you, before I heard your voices — I liked them: nice little voices; they reminded me of something I cannot remember — if I had seen you before I heard you, I should have just trodden on you, taking you for little Orcs, and found out my mistake afterwards.
I can see and hear (and smell and feel) a great deal from this, from this, from this a-lalla-lalla-rumba-kamanda-lind-or-burúmë. Excuse me: that is a part of my name for it; I do not know what the word is in the outside languages: you know, the thing we are on, where I stand and look out on find mornings, and think about the Sun, and the grass beyond the wood, and the horses, and the clouds, and the unfolding of the world. What is going on? What is Gandalf up to?
Both are quotes from Treebeard in The Lord of the Rings, book 3, chapter 4.
- What does it actually say?
Let's analyze the line. It begins by saying
I always like going south
which tells me that this is something Treebeard has, and did when speaking, felt or believed. In other places than when he mentioned it the scene.
Again, although we lack context for the line originally, Peter Jackson on the commentary told us that it was not purposed for the scene in which it was placed: it was simply something that John Rhys–Davies said and which Peter, Philippa, or Fran wanted to insert into a scene as if it were ‘additional dialogue recording’.
It continues with
Which is to say that Treebeard has not discovered how exactly it does, but simply that it does.
it feels going downhill
What would an Ent feel while going downhill? Unfortunately, I know of nothing which would give us much information there.
Because I know of nothing which would imply that John Rhys–Davies was an eminent scholar of Tolkienology, I cannot suppose otherwise than to say that the line was mostly gibberish.
Meaning no discredit to John, of course. If anything, the blame lies on the editor who inserted that line into the finished movie. (Of course, that's my personal take on all this.)
So, to conclude this answer:
Is there a difference in elevation or something that can justify Treebeard's feelings, or is this just some random feeling he feels?
Probably the latter. The rendition of Treebeard in the Peter Jackson version of the story seems to imply that he is rather — shall we say, senile.
I mean, maybe it was included so as to help explain why Treebeard would believe Merry's rather ludicrous request that he carry them south at all.