The Nissan Path

I have a friend with a 2005 Pathfinder SE with about 150,000 miles on it, and about two weeks ago, it began to shake/vibrate at around 40-45mph and up. The noise acompanying it is almost identical to that buffeting noise you get when you roll down a rear window by itself, but a bit faster and not as loud; the vibration is in sync with the noise.

I've narrowed the source down to the rear differential (test drove it myself, putting it in neutral to take the load off the drivetrain, looked at the tires for signs of separation/unusual wear patterns, spun them all freely by hand, listening for wheel bearing noise). Upon getting the rear end off the ground and spinning the wheels by hand, I can hear a fairly faint noise that sounds identical to a bad bearing, and when I put my hand against the rear diff, I felt a little bit of a vibration, again, similar to how a bad bearing would feel.

My questions to you guys, as Pathfinder enthusiasts, are these: are there any bearings that may have gone bad? Would a simple fluid change take care of this? Has anyone else even experienced something similar? I tried a Google search, which led me to this site, and I tried a search specific to this site with no luck. My apologies if this has been discussed and I missed something.

It's possible that there could be bad bearings. If they are bad, changing the fluid will not help them; the bearing(s) will need to be replaced. I haven't heard of any differential bearing or axle bearing issues with the R51s, but that doesn't mean they don't exist nor mean that yours aren't bad. Most of the vibration issues on the R51 are usually due to bad U-joints or automatic transmissions that have been contaminated with coolant, due to a failed trans cooler inside the bottom tank of the radiator.