I'm going to agree with Dragonlady on this one - that sounds like internal energy.
Could you give any more details about this, like where in your back it starts?
The only clues I can offer are that the chakras are generally considered to be in a column along the spinal cord (and therefore the back), and the hands and feet are considered entrance and/or exit points with the discrepancy (whether it is an entrance, and exit, or both) seemingly depending on what energetic tradition you ask. If the vibrations start from a specific point on your back it could indicate that a particular chakra is discharging large amounts of energy, which then courses through your energetic system and ends/exits in the hands and feet.
If it's the entire spine that would make it sound more like it's an issue of kundalini (the metaphorical snake which awakens in your root chakra and stretches up through all of your chakras. Kundalini work is much more advanced than basic chakric knowledge though, but I believe I've seen a few members here post articles about it so I imagine someone here would be of help if this turns out to be the case.