Lynn,

Thanks for being available on the board.
My question is regarding string crossing in thumb position:

I do not have a classical background and sometimes feel melodically limited in that register..Tips? Basic guidelines?/etc..

Oh yeah....I love that Peacocks tune...
Happy Holidays..

I suggest getting with a good teacher. They can help you with posture and fingering. There are many good books that will help you learn thumb position. A few of them are: Rufus Reid's Evolving Bassist, Michael Moore has a thumb position book, Rabbath's New Technique books.