SRS posted while I was putting that together.

Show me a single example of "jubal" used adjectivally anywhere in the English language (or in any of the senses you suggest, other than simply as a name) and I might reconsider. So far, I haven't seen a single piece of real evidence in all this elaborately-constructed speculation. The song doesn't appear to be particularly old; hundreds such were written during the early 19th century. Many contained classical allusions of the conventional sort (Phoebus shows up a lot) but I can't think of any Biblical ones at the moment. Where nominally told from the fox's point of view (though really from that of the hunter), such songs are still couched in conventional terms; don't expect realism!

What is far more common than deliberately obscure language (especially terms not found anywhere else) in folk song is the case of the singer who mishears certain words or phrases in a song at the time of learning it. Typically, they will either substitute words or phrases that do make sense to them, or they will simply sing the sound they remember. Such limited evidence as we have is strongly in favour of the last possibility in this case.