It sounds like you have located your problem in the rear of the car, specifically the space saver.  Please don't take it to Nissan, it will be a waste of your money. 
  
+1 - Best option is get someone from on here whio knows their way round the 350's and passenger you.  Not sure where you are based, but if you let on then I am sure you will get some help and a drink comes an awful cheaper that £100 an hour.  Might regret saying that as someone will say they can down more than £100's worth in an hour