Weird squealing noise

I've been have a strange noise and would love some help troubleshooting.

The Truck:
2001 Dbl Cab Pre-Runner 4 Cyl bought at 40,000 miles and currently 110,000.

The Problem:
When maintaining speed (> 25 mph) on nearly flat ground there is a sometimes barely audible other times deafening squeal coming from under the truck and seems to be under the cab. If you add a little more throttle it goes away. If you let off the throttle and coast, it goes away. It is a high pitched metal on metal sound that makes me think of nails on a chalk board in fast forward. Within a few months of buying the truck (years ago) I heard it for the first time. Has been off and on since then. I may hear it every time I drive for a week and nothing for a month. I also may hear it for 1 min on a drive or for 5 hours non-stop. There is also no variation between weather or seasons.

Fix Attempts:
I've had it to multiple Toyota dealerships on multiple occasions. I've only been able to reproduce the sound 2 times in the presence of a mechanic. Both mechanics more or less blew me off and said it was brakes. The brakes are fine on the truck and this has been spread out over 6 years and multiple sets of brakes. Visual inspections haven't turned up anything either.

Any ideas would be greatly appreciated. Thanks

 

Any vibration occuring when this "squeal" takes place ? & does it make a low level "Howling" pitch type of noise ? When was the last time your truck's driveline & chassis were lubed ? (by someone who KNOWS where all the lube points are) Grease it all anyway, crawl underneath your truck (if you are mechanical in nature) & with a good light , thoroughly inspect the area around the center (carrier) bearing,its non-servicable, it may be bad, & it could also be the "double Cardan joint" right behind the carrier. If this is bad, its driveshaft time Bro', at least this gives you a place to start looking, its not uncommon for a carrier bearing to intermittantly "freeze up" & squeal like a pig when its life is coming to an end, hit me back & let me know what you find......

 

The sound kinda makes an eeeeeeeeeeeeee sound. Not sure when it was last lubed but am sure its been awhile so I'll get that done. Would it be possible for a carrier bearing to periodically freeze up over tens of thousands of miles and half a decade? I always figured that whatever it was would get worse and present itself one day but it isn't any more frequent today than it was 6 yeas ago. I am fairly mechanical but am not sure what to look for in a bad carrier or double cardan joint. What kinda things should I look for? Thanks