The pinion coupler from the rear center driveshaft needs to be moved as far back as possible on the pinion shaft. The coupler has a little raised portion that is supposed to ride near the bearing for the rear pinion. The rear diff needs to be taken out so you can push the pinion gear forward and then attach the pinion coupler to the pinion shaft from the outside of the diff case. It doesn't need to be smashed against the bearing.....it just needs to be up against the bearing.
Also if these are new vehicles run them for several tanks and they will free up. This question is brought up about every two weeks on this thread. if you read back a few pages you will see how many times it gets asked. LOL. So you don't have anything to worry about. Enjoy the truck.