2002 Infiniti QX4 Vibrations!!

[brickbox]

I recently bought a used 2002 QX4 4WD with 143K miles on it.
It seems to have a vibration that I am unable to figure out why. I just had 4 new matching tires inflated to the same PSI installed yesterday. But the problem still persists.

Its very smooth until about 48 mph. Right around 50mph the vibration gets pretty bad. If I switch it from 2WD to Auto - the vibration goes away for a few minutes and then comes back. No change switching from Auto to 4Hi.

The vibrations feel the same if I engage/disengage O/D. Also, its the same if I shift out of "D" into "N".
In 2WD, the vibrations reduce at about 60 mph and is present at 80mph. It gets really bad at 85mph. If I switch at these speeds to Auto or 4Hi - then the vibration is pretty bad. Letting off the gas helps a little bit, but its still there.

I crawled under the car and the drive shafts seem solid. I didnt notice much of a play (maybe 2 mm) when I press around the center support bearing.

As far as I know, the diff fluids have never been replaced on the car. I plan to take it to the shop next week and get new fluids in both the diffs.
Anything else that could be causing this?

Thanks!

[rgk]

It could be several things: driveshaft(s) out of balance, wheels out of balance, diff fluid.

You really shouldn't switch from Auto to 4Hi at those speeds.

[brickbox]

Thanks - I read the owners manual and it said the same thing. I just did the switching to 4Hi once. After that, its only 2WD to Auto. I took it to another shop today and had the wheels balanced again. But no difference. Additionally, I noticed that the front diff/trans is bit whiny in 4Lo going about 15mph - is this normal?

Do the driveshafts spin even if the vehicle is in motion and the gear in Neutral?

[x2002Qx4x]

Driveshafts are spinning if the car is in motion, even in neutral

[Rockwood]

I would just say that these u joints have no grease zerks, and after all these years, the grease in them can get old and hard. The joints get a rough spot or flat spot, so even though it feels tight when you check it, it has a problem. Only way to find that is to remove the driveshaft, and check the movement of the ujoint by hand for roughness.

You can remove the front driveshaft to see if that is part of the problem - just stay in 2wd.

[brickbox]

Update - so based on what you guys said, I took the front drive shaft off. The u-joint was in pretty bad shape. So I had a local shop replace it for $240 (had them do both the u-joints while at it).
That fixed the clunk. I still have vibrations at highway speeds in auto but only when I am on gas. Same story at 50mph. It there's only when I am on the gas.

Today I take the car in for the airbag recall and to have the diff fluid replaced (dealership was the same cost as a local shop!).
If that doesn't fix it, then I intend to take it to a driveline shop that quoted me $200 to get both the front and rear driveshafts balanced.

[brickbox]

So update: After spending about $700 - I got the problem fixed. Diff. fluids being changed didn't help in any way. But at the very least, I know I have some time before I need to flush them out again. All 4 u joints had to be replaced and the front and rear drive shafts were balanced and re-installed. Drives silky smooth with no vibrations.