Usually, you can give your VIN number to the Infiniti dealership, and get the right part.
I don't trust the local auto parts store for important parts like half shafts. 90% of the ones I have bought over the yeras for Hondas and Nissans were crap, and didn't last. 100% of the struts I ahve bought at these places failed within the first month. There is nothing more frustrating than doing the labor twice because of inferior parts. OEM parts are not that much more, and are well worth the extra cost.
Check out
http://www.everythinginfiniti.com If you want to talk to someone, ask for Joe.