I have a 2003 xterra and i have a whine/howling noise at 55 mph. the nissan techs thought it was a diff, or wheel bearing replaced both and still makes noise. the noise comes with the throttle when you tap the throttle at 55 it gets louder and when you let off it gets quieter. if you have any solutions or comments please let me know.

Could be a few different things including carrier bearings or the pinion bearing. Could also be worn or improperly backlashed ring and pinion gears. Have you checked your diff oil for shavings, etc?

I didn't find any more shavings than normal. do you think that the backlash could be wrong on two differentials, when i replaced the diff the noise was exactly the same. is there a special type of fluid i can try to quiet the noise?

No, that's not likely. Since I don't know what's in there, I'd recommend a good 80w90 synthetic gear lube. Will cost you about $20 to drain and refil if you do it yourself. It takes just under 3 quarts. Be sure to loosen the fill plug first to ensure you can get it broke loose before you drain the diff.