Hey guys,
I have this exact same symptom and need some input to solving it. The noise does not change pitch when under load or decelerating. It sounds just like I have a set of mud tires on the truck when going down the road. I put the rear on jack stands and the noise only increases in loudness when the speed increases. The sound appears to be from something rotating at a higher speed than the wheels (driveshaft/pinion).
There isn't a grind to it, only the whirring noise and it happens under any condition. The noise does not change pitch when one side is loaded more than the other (turning). The driveshaft u-joints do not have any play in them. Rear diff fluid is up to the fill hole.
Used the screwdriver stethoscope method and the noise is coming from the center of the diff (no difference for L/R carrier bearings or pinion bearing). Axle shaft bearings are relatively quiet.
Also makes same sound in reverse.

Any input is appreciated.
2006 Ford Ranger XL 4x4, 4.0L, 4.10 rear with LS.
Thanks,

 

Seems to be classic symptoms of a pinion bearing going out for anyone searching this.

 

Rebuilt rear end. Pinion bearing was rough. Went ahead and replaced pinion bearings, carrier bearings, axle bearings/seals and she's quiet as a mouse now. Mechanic cost $1150, turn-around 1 day.

 

Was the replaced bearing pitted... did you get a chance to look at the parts ?

Is your breather vent tube connected to the axle tube ? Sometimes water gets in via a cracked or broken Breather Tube.

 

$1150? Holy ****! I paid $300 in labor to have rebuilt with a master rebuild kit. I supplied all parts and materials.

 

Good for you. Buying your own parts is indeed a good way to save money. I honestly don't mind paying for quality work when it comes to differentials & transmissions.

 

It's intact. Thanks for asking because I had not checked that. The inner pinion bearing was indeed pitted severely and rough/loose and there was a pile of fine shavings. I replaced them all including axle bearings because it's my DD and I drive A LOT (about ~500 miles a week).

 

You got it done... should last for a while !

Ltr