I spent some time looking how to condition arbitrary voltages say from 1.5v to 2.5v to convert it to 0-5V for Atmega inputs. Is not the case for the +/- differential strain gage but the scaling part will works for you.
One of my last circuits was to scale a +/-5V to 0-5V but I repeat, is not differential, just bipolar to 0-5V.
To convert differential to single ended you must use a instrumentation amplifier circuit.
Could you post your schematic?