Loud Squealing from under 2003 Toyota Prerunner

Hey guys-I have a 2.7l Toyota Tacoma Prerunner SR5 Extended cab bought new in 2003. Its been a great truck and just passed 190000 miles. I have been dealing with a horrific loud squealing sound coming from under the truck. This started off way back when the truck was still under warranty but did not do it enough to cause issues. It does it worse running flat, off load-you cannot hear it over 60 miles per hour. It does it mostly when I take my foot off the accelerator. After driving for long distances (30 min or more on the highway) it is worse. Something heating up? 2 toyota dealerships have not been able to figure it out. Things ruled out-belts replaced with tensioners, wheel bearings, driveshaft (u joints and carrier bearings). Everything underneath has been lubed. Latest dealership thinks it has to do with back drum brakes. When they adjust them the sound goes away for a week or two then comes back. They did say brake pads on drums still look brand new (is that normal for a truck). They have never been replaced. Any thoughts? Thank you-new to the forum.

 

Both times my carrier bearing failed it was pig like squealing that varied with load.

 

Thanks-I just find it hard to believe that it would be doing this for over 150000 miles on and off. Both toyota places seem to think drive shaft and everything else are fine. Frustrating to say the least...