Also had starter issues when I 1st got my Jeep, a real PITA! High pitch grinding noise and difficult to start. My starters used to break, e.g. nose cone bust off, gear broke. In the end I replaced the ring gear, even though the previous one didn't look to bad. Seems like once it starts grinding the ring gear does not last long.
If you have the alignment dowels and you've already replaced the ring gear, then the only thing that comes to mind is that starter solenoid and/or wiring to starter, maybe there is not enough juice to fully engage the gear. Try painting the starter gear, and then do a start. Pull the starter, and see how much contact you are getting.