I believe there are two different reasons for each of the lines you quote, despite them coming from the same song.
Remember this is a comedy rock song - so don't take it too seriously.
We was hitchhikin' down a long and lonesome road
You are correct that the proper grammar would be "we were hitchhiking...". This is an example of colloquial speech, imitating a particular kind of localised American dialect. This kind of language is popularised in country music and so I believe this is used here to invoke that kind of imagery, as parts of the song appear to be a pastiche of country music with the story-telling lyrics and acoustic guitars.
Be you angels?
Again, you are right that this is not grammatically correct in modern English - but it is in imitation of Old English, or Early Modern English, the kind sometimes used in folklore tales. In this particular song these words are supposedly spoken to them by a demon they meet in the road. Early Modern English is the language used in the King James translation of the Bible - for example at Ephesians 4:32 the old English translation is "be ye kind to one another" - so I think they used it to invoke this kind of imagery.