Character sword on back equip

First, you need a gameobject on your characters hierarchy that matches your requirements for a proper animated position on the back of the character (could also be the torso directy, if the actual back does not move relative to it too much, like through skinning. Then you'd need to place another bone to the back, staying with it).
I'd add another gameobject to it, placed in the exact spot and rotation the sword would need to be. parenting it via could then is just setting the localposition to 0 and the rotation likewise.
For the bouncing, just code a little up and down, reading from the speed the character is animating its walkcycle with.
For the pick and put back animations, just make sure the hand of the character reaches for the tip of the sword on the pack and the moment it's either of the situations, attach or detach the sword from/ to the hand/ back socket.