Reading 5v analog to 3V PIC
Just connect the 2 grounds of the different circuit (and use the voltage divider of course) and you'll be ok. It ensures that the divided 5V from the sensor is a meaningful voltage for the PIC. The PIC won't notice anything about the 5V in the other circuit.

edit
OP seems to need isolated circuits (was not in the question). There are analog optocouplers which allow you to get your sensor voltage across an isolation barrier. The IL300 is mentioned a few times here on electronics.stackexchange, for instance here. You won't need the voltage divider.