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What my truck experienced was not a squealing, but more of a groaning as seen in the video below. This turned out to be air in the hydroboost system that did not present itself until the ambient temperature dropped below 30° and about 6 months after replacing the steering gear. The vibration that accompanied the groan could be felt in the steering wheel and the brake pedal.
I agree with Sous' assessment of air in the hydraulic circuit. Did you raise the nose of the truck off the ground and steer lock to lock while pulsing the brakes with the engine running after you replaced the hydroboost unit? Is your PS pump reservoir at the correct level? Did you use ATF?
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