Joe,
there is not enough information to make a diagnosis of your problem without seeing or testing your vehicle.
Most squealing comes from the front brake pads, not the rear. Also, your Parking brake is separate from the rear brakes. The rear brakes are disc brakes and use calipers and brake pads similar to the front brakes. However the Parking brake is a small drum-style unit inside the hub of the rear brake rotor disc and do not share any mechanical parts except the rotor and the rear brakes don't really have that much stress or strain on them.
Since you said it was a new truck, take it back to the dealer and let them figure out what is wrong. I am suspecting that there maybe something loose or dragging in the parking brake assembly, or the rear disc/caliper assemblies? ....This is just assuming that the squeal is comming from the rear wheels, which is very unusual in itself?
My advise is that if you cannot diagnose your own brake problems, you probably should not be trying to fix it yourself. Anybody can slap in a new set of pads, but you really need to know how the parts work and interrelate to each other to know what causes the various brake problems, and how to fix them.
Too much automotive repairs are being done by simply replacing parts without really knowing if that is what is causing the problem. Dealerships are deemphisising the diagnostic aspects of automotive repairs to get quick fixes and get the customers money and get the vehicle out of the door. If it did not fix the problem, they will be back and you can charge them again for the labor and a new part.
...Rich