Nissan Repair: Differential, differential assembly, spider gears

Question
I have a 1999 Nissan Frontier King Cab 4x4 with a v-6 Automatic.  When i start to move forward in the truck while turning to the right there is a loud noise in the rear defirential that sounds as if the teeth on the gears are not matching well.  However, when i start to move forward while turning to the left I hear no noise at all.  When I am driving down the road there is small vibration as if the drive shaft is out of balance, otherwise noise only when I turn to the left.  Can you give me any advise as to what may be going on.
Secondly, I have found a comeplet rear differential assembly with axles and all in good condition.  The problem is i am not sure it is the right ratio.  Is there any way to know the ration of the used rear end without taking it apart and counting the teeth?

Answer
Billy,

Sounds like the spider gears in the differential are blown.  I would go with the replacement.  Chock the tire and the driveshaft or flange and rotate the tire a complete turn and count how many turns the driveshaft or flange goes.  If it is three turns to one tire turn you have a 3.00:1 differential.  If it goes two and three quarters turn it is a 2.75:1 rear end.  You get the idea.