Click & Drag change from xy to xz

So I've done up a script with allows the player to click and drag objects with the mouse currently at the moment it moves on the XY axis however I want to make it so the object can only be set to move on the XZ axis. How would I go about doing this? I also have made an snap to ground script so that if the object collides with another object it will snap to the surface of it.

MouseScript

 using UnityEngine;
 using System.Collections;

 public class MouseScript : MonoBehaviour {

 private RaycastHit rayHit;
 private GameObject collideObj;
 private float distance;
 private Vector3 posObj;
 private bool lockObj;
 private int layerMask;

 // Use this for initialization
 void Start () 
 {
     layerMask =~ (1 << 8);
 }
 
 // Update is called once per frame
 void Update () 
 {
     if (Input.GetMouseButton (0)) 
     {
         var ray = camera.ScreenPointToRay (Input.mousePosition);
         var hit = Physics.Raycast (ray.origin, ray.direction, out rayHit, 100f, layerMask);
         if (hit && !lockObj) 
         {
             lockObj = true;
             collideObj = rayHit.collider.gameObject;
             collideObj.GetComponent<StickToGround>().enabled = false;
             distance = rayHit.distance;

             Debug.Log (collideObj.name);
         }
         if (lockObj) 
         {
             posObj = ray.origin + distance * ray.direction;
             collideObj.transform.position = new Vector3 (posObj.x, posObj.y, collideObj.transform.position.z);
         }
     }
     else 
     {
         lockObj = false;
         if (collideObj != null)
         {
             collideObj.GetComponent<StickToGround>().enabled = true;
             collideObj = null;
         }
     }
 }
 }

Stick to Ground Script

 using UnityEngine;
 using System.Collections;

 public class StickToGround : MonoBehaviour 
 {
 private RaycastHit rayHit;
 public float deltaPos;
 // Use this for initialization
 void Start () 
 {
 
 }
 
 // Update is called once per frame
 void Update () 
 {
     var hit = Physics.Raycast(transform.position, transform.up * (-1), out rayHit);
     if (hit) 
     {
         transform.position = new Vector3(rayHit.point.x, rayHit.point.y + deltaPos, rayHit.point.z);
     }

 }
 }