I am no expert, but wouldn't something like this work:

Take an opamp.

To the - input connect one end of the shunt via a precision resistor of
value R
To the - input also connect regulated V+ via another precision resistor of
value R

To the + input connect the other end of the shunt via a precision resistor R
To the + input also connect regulated V+ via another precision resistor of
value R

Without any further wiring, we would now find both inputs of the opamp to be
biased to a value very close to 1/2 V+.

Now we add an appropriate feedback resistor to get the required gain to
cause the opamp output to swing through the good range for measurement on
the PIC.

Bob  Ammerman
RAm Systems

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