Why do I have low voltage and backfeeding between circuits throughout my whole house?
Very very dangerous. First, all your electrical equipment is at risk.  Shut off the mains, call an electrician.  Or wait a bit longer and call the fire brigade.

If you have a shared neutral, or multiwire branch circuit, the neutral could have broken.  Or the house was miswired (see 
http://www.homeinspector.org/resources/journals/Multiwire-Branch-Circuits.pdf ). Or what should have been a double breaker flipped, leaving the other half hot.