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 .

It only happens when I pretty much floor the accelerator and stops when it kicks into gear. A shop said they couldnt replicate it on the lift but maybe they didnt floor it. They definitely heard it while driving. I definitely believe it to be something with the differential but could be wrong.

Ill try and get a video with sound. I dont know if my phone would be able to pick up the sound though

No luck with the sound. It picks up more road noise than the noise itself. What I dont understand is, if it is a bearing, why would it only happen under this situation of flooring it until in it hits gear as opposed to all the time since they are spinning

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 )