I would jack the truck up and rotate each tire individually. You need to try and narrow things down a bit. You've eliminated belts etc. Wheel bearing maybe?

 

I may well be wrong, I am NOT a brake guy by any means, but don't the pads squeal whether the brakes are applied or not when the "squealers" are in full contact?

But yeah, now we're on to wheel bearings, and such. Rear brake shoes another possibility? If they're not adjusted properly they can drag. Maybe from that?

Just a few thoughts...
Pat☺

 

Good idea, better try that tomorrow

 

Yes, I suppose the brake pad warning tab can rub (even when not applying the brakes) once the pads get really really low. However, front pads were replaced 10,000km (6000mi) ago. Still worth having a look though

 

Ended up taking truck to my mechanic. As luck would have it, the squealing stopped that day (and that day only). Nevertheless, based on what I told him he checked a number of things - belts, brakes, wheels etc. He also lubed the u-joints. However, the sound has returned albeit not necessarily as much as it once was. At the moment it squeals for a while (and not terribly loud) when I first start out for a few km. Could it be related to differential? Again, the squeal (when it exists) has nothing to do with the engine revs but does have to do with the turning of the wheels. If I am going down hill I can put in neutral (reduce rpm to idle) and the squealing sound does not change at all. In fact, I can turn the engine off and it does not change.