Metallic screeching sound from back end

[amarotica]

Hi all, my truck is making a really aweful metallic screeching sound when I first drive in the morning or after sitting for an hour or more. It's not a belt or brakes I don't think because 1. It doesn't do it when stationary and 2. It only screeches for about the first 30 seconds of driving away from my house at 10-30mph. After about 30 seconds it stops which leads me to believe SOMETHING needs lubricated as it just started and got worse over the last two weeks. I will change fluids but I wanted to see if anyone could suggest a place to start... This is what I am thinking it could possibly be:
Driveshaft U joints grease
Transfer case oil
Differential oil
Rear axle grease

Also it seems to be louder when I press the gas or "engine brake" and when I shift to neutral or press the clutch it gets slightly quieter. I'm 99% sure it's coming from the rear or center of the vehicle behind the drivers seat (cab) and NOT the front.

Thanks in advance for the help!

 

[Shran]

U-joint with a dry cap would be my first guess. Put the trans or t-case in neutral, chock the wheels, and see if there is any play in the rear driveshaft u-joints. It might be a small amount of movement or a lot depending on how many needle bearings have turned to dust.

Rear brakes would be my second guess, the Astro van I drive at work had a similar issue. In the morning it would make a godawful amount of noise when moving and the brakes would actually lock up if you touched the pedal in the first 100 feet or so of travel... super annoying but after that it was fine all day. Turned out that the brake shoes and drums were totally worn out.

I don't know if your year has a 2 piece rear driveshaft with carrier bearing or not but that would be my 3rd guess, bad bearing.

 

[Rex2nr]

Check your heat shield. I just changed tranny and transfer case fluid, gear lube, and pinion seal. I thought I over-tightened the pinion and killed the pinion bearings, but I actually knocked the heat shield loose and it was dragging on the driveshaft and screeching. I went straight to the u joints and pinionbearing, then found that.

 

[amarotica]

Thanks for the suggestions

Thanks to all with the suggestions! I went and got some gear oil and WD-40 cause I thought the sound ended up coming from the rear differential but before I opened it up, I decided to spray the U joints with WD-40 and the noise stopped immediately! I guess the grease I tried using on the U joints before just didn't penetrate far enough into the system... Stupid me for 1. Not having WD-40 already and 2. For not trying that in the beginning!

 

[Doofy]

I doubt WD40 will solve anything for long. U-Joint damage may have already begun so be vigilant.