So, I took a road trip this weekend and something happened to my Xterra that had never happened before. As I was leaving the Interstate and downshifting from overdrive, my car started making a weird grinding noise. I had the clutch pushed in completely and as I was doing the downshifting, I could feel the shifter vibrating in my hand. As I said, it had never done this before and it actually quit while I was driving in town. That was yesterday, but It did it again today as I was exiting the interstate and slowing down from a high rate of speed. I checked all my fluids, but I figure if it was a fluid problem it would do it on every shift and thats not the case here. I have an 02 Xterra with about 63,000 miles on it and have it looked at every 3,000 miles when I get the oil changed. Any Ideas as to what could cause this?

How old is the gear lube? You're supposed to change it occasionally. I just did mine on my 06 and used a good quality 85w-90 gear oil. It made the shifting smooth out for me. It's real easy, there's a filler plug on the side and a drain plug on the bottom. Get yourself a hand powered suction gun from NAPA, screw the top off and fill it, then put the hose end in the gearbox filler hole and push the plunger down. Dumps almost a whole quart in one push.