What would cause a squealing noise when accelerating fast but stops when it Kicks into gear?

Possibly a rear pinon bearing, or if its a two piece driveshaft a carrier bearing which is in the middle of the two halfs.

You could jack the truck up in the rear support it on jack stands and place the truck in neutral and spin the rear tire and see if you can duplicate the sound. But my guess is once of those two items but its hard to say without seeing or hearing it.

Maybe play within the drivetrain, also flooring the accelerator puts a lot of strain on the components in the system, you could also look into U-joints at this point ( maybe bearing caps are binding up )