Could be several issues, after to replaced the starter when you started it did you hear a high pitched whine when you let off the key? If so your started would need shimmed. Did the teeth grind when you first turned the key? If so then the teeth were not engaging properly. Are you sure you are getting the right number of teeth on the starter gear? There should be 9 teeth on the gear if you have a 4.2.
This is my Jeep. There are many like it, but this one is mine.
My Jeep is my best friend. It is my life. I must master it as I must master my life.