Assign prefab to variable through script

Now I've looked a heck of a lot for an answer to this but have yet to figure it out.

What I am trying to do is change what is assigned to the prefabObject variable depending on what button has been pressed, this is my code which shows my attempt at this. I also have my prefabs named "wall" and "woodenPlank" in a folder called Resources.

 var selGridInt : int = 0;
 var selStrings : String[] = ["Grid 1", "Grid 2", "Grid 3", "Grid 4"];
 var showInventory = false;
 var cursorLock = true;
 var disableCamera = false;
 public var prefabObject : GameObject;
 private var offset : Vector3;
 private var fwd : Vector3;
 private var clone : GameObject;
 var unit : float = 3.0f;
 var Sound : AudioClip;
 
 function Update () {
 
 fwd = transform.TransformDirection(Vector3.forward) * unit;
 offset = transform.position + fwd;
 if(Input.GetKeyDown(KeyCode.E)){
     
 audio.PlayOneShot(Sound);
 clone = Instantiate(prefabObject, offset, transform.rotation);
     
 }
 
 if(cursorLock == true){
 Screen.lockCursor = true;
 }
 if(cursorLock == false){
 Screen.lockCursor = false;
 }
 
 if(disableCamera == false){
 gameObject.GetComponent(MouseLook).enabled = true;
 }
 if(disableCamera == true){
 gameObject.GetComponent(MouseLook).enabled = false;
 }
 
 if(Input.GetKeyDown(KeyCode.I)){
 
 disableCamera = !disableCamera;
 cursorLock = !cursorLock;
 showInventory = !showInventory;
 
 }
 }
 
 var wall : GameObject;
 var woodenPlank : GameObject;
 
 function OnGUI () {
 
 if(showInventory == true){
 selGridInt = GUI.SelectionGrid (Rect (200, 100, 300, 300), selGridInt, selStrings, 2);
 
 if((selGridInt == 0) && showInventory == true){
 wall.SetActive(true);
 woodenPlank.SetActive(false);
 prefabObject = Instantiate(Resources.Load("wall"));
 print("wall selected");
 }
 
 if((selGridInt == 1) && showInventory == true){
 wall.SetActive(false);
 woodenPlank.SetActive(true);
 prefabObject = Instantiate(Resources.Load("woodenPlank"));
 print("wooden plank selected");
 
 }
 }
 }