1) pls cut the profanity
2) Of course you need to have a (floating) common. BUT when the signal common floats, you can have a ground potential difference between transmitter and receiver of up to what is specified in the datasheet (I seem to remember once using a galvanic chip that could handle 60V). Still with a floating common, that should be a wire and if dirt (ground/earth actual ground, not datasheet gnd) is available the shield should be connected to dirt in one end. I can only guess what value (positive, negative, none) a shield would have if not connected to dirt.

Erik