I drive a 2001 Xterra with 80k miles on it, and I dont quite know if the problem is with the transmission or the clutch? I have absolutely no problems with shifting...everything works fine. However, when I'm about to downshift with the clutch depressed, and the gear selected, you hear a grinding noise form the front of the vehicle. This is only while the clutch is depressed and the gear is selected. When clutch is released and you actually shift into gear, you dont hear anything. Also, when you just depress the clutch without a gear selected (in neutral), you don't hear anything.

My Nissan Dealer says I need a new tranny for $3,500. My AAMCO mechanic says it's the clutch. I'm lost!

Well first off this is a J-body forum but still, my guess would be the throwout bearing on your clutch.