1996 f150 2wd 8.8" 3.55 rear end noise what is the issue
 

https://youtu.be/55HCCBCVSfw


This video was taken an hour ago. It's my 1996 f150 5.0 2wd 8.8" 3.55 rear end. Is the noise coming from pinion bearing, u joint, etc.... It only happens at low speeds 10-35mph. It gets worse after it shifts into 2nd gear

Sounds like a creaky suspension to me. That noise does not sound like it's u joint or differential noise to me. It's so suttle I am surprised you can hear it while driving.U joints will show powdered rust on the spider when the lube has been depleted and the needles are disintegrating. I don't see that with yours.

It's loud enough if I were driving down a city road everybody will hear it outside and inside the truck.

The noise does sound like it comes from the diff

Did you watch the entire video with the volume all the way up? It's a noticable noise.... Sounds like chains dragging on ground. The noise is coming from the opening of driveshaft. Nothing with the suspension

I had a difficult time picking up the noise. Turning up the sound to hear it tells me it is not that pronounced.

Disconnect the driveshaft and make sure the u joints are smooth and not rough or binding. If that checks good, verify the differential is filled with oil. Rotate the pinion flange with wheels, drums or calipers removed to see if there is any roughness or noise. Usually differential bearings and gears make a whine at various conditions. Otherwise, seek a professional opinion at a qualified shop. Many stay clear of differentials because it is difficult to set them up properly and be noise free.

Noise
 

At the very end of the video I can see the pinion move in and out, it should not do that, you need to rebuild the diff.

If the pinion moves in and out, the bearings are shot OR the nut holding the pinion flange backed off some. If the lube level is where it should be .the pinion seal is shot too and should have leaked out.

No fluid has leaked out.

Something of note: every time it rains the noise goes away.... Makes me think it's something outside of the diff that gets wet and the noise stops...

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Not going to rebuild. Would just put a new one in. But I'm not convinced it's the diff.

The noise goes away everytime it rains. The water get onto where the noise is coming from and stops it...

The very least you should verify your pinion preload is set to specs.It's more than just torqueing the pinion nut and it's about an hour job.