Motorcraft ZD-11 have been superseded to ZD-30

These are OEM - dealer - factory direct glow plugs - beware of aftermarket look a likes that are NOT OEM and are usually priced much cheaper. 

Price is PER glow plug - 8 required per engine.

The following video is for 7.3's - how to test your glow plugs through your harness. 

The following video is for 6.0's but it is still relevant for this product.