Try jacking at least one front wheel, off of the ground before trying to move the 4x4 rail again - if the vehicle has been moved on a hard surface with 4WD engaged, there will be some degree of drive train "wind up" that will make it difficult to move the 4x4 shift fork - allowing one end to spin freely will relieve the binding.
As you're new to the Samurai you may not be aware that it is what is known as "part time 4WD" and the 4WD modes should not be used on a hard dry surface - if there is no slipping, the drive train will bind and it's possible to damage it.