This doesn't really answer your question, but why don't you just delete the RigidBody from the ground and then add a KinematicBody2D version of the rock in the character's hands? When you want to throw the rock, you can either swap it with the RigidBody version again or use
move_and_slide(). So the whole sequence is broken down into 3 parts:
1) Pick up the rock. The rock disappears from the ground and a KinematicBody2D version of the rock appears in your character's hands,
2) When about to throw the rock, delete the KinematicBody2D and spawn a RigidBody rock in its place.
3) Apply forces to the RigidBody rock so that it moves forward or whatever.