I had a simmilar problem with my wife's 96 Rav-4. We bought it used, and a few months after we had it, the brakes began to squeel VERY LOUDLY. It was only when we were going a few MPH, right as we were coming to a stop. So bad it was embarrasing to drive. It was the front brakes (rear are drums)

The pads were probably 50-70 remaining. I took the brakes apart, lubed everything up really good with the proper brake lube, and lightly sanded the pads and rotors so the pads would re-seat. That worked great, for a week. Then they started the exact same squeeling. The pads were OEM Toyota. I finally got fed up with it, and bought some $20 pads from the parts store, and that took care of the problem, for good. I lightly sanded the rotors, but did not replace them.

Is your truck still under warranty? If so, I'd go back to the dealer and demand new pads.

Otherwise, replace the pads and lube everything up good, and that should fix your problem.