Is there a way to differentiate between left and right Shift keys being pressed?

You can do it like this:

-(void)flagsChanged:(NSEvent *)theEvent {

    if ([theEvent modifierFlags] == 131330) {
        NSLog(@"Left shift pressed!");
    }

    if ([theEvent modifierFlags] == 131332) {
        NSLog(@"Right shift pressed!");
    }
}

In Swift:

let isLeftShift = event.modifierFlags.rawValue & UInt(NX_DEVICELSHIFTKEYMASK) != 0
let isRightShift = event.modifierFlags.rawValue & UInt(NX_DEVICERSHIFTKEYMASK) != 0