Truck started making a weird squealing noise today under acceleration/load. Turns out its coming from the rear diff/pinion.


I had to hold my foot on the brake to simulate a "load" during the video to get it to make noise. It doesn't make the noise in reverse, or just coasting.
Any ideas what could be making this?? I sprayed some WD-40 on the u-joint with no change...

 

After a lot of searching, it appears that this is very likely caused by a bad rear u joint. I just find it weird that it made literally no noise until today!