Que hit the nail on the head. I just had a few similar problems (charging at 19v and not charging). For me, the first fix was a wiring short.
Obviously, to start, check your fuses at the PDC. If all the fuses are good, check grounds. The ground i am specifically thinking of is the ground molded into the black box connector that attaches to the alt.
After testing the alt, if it comes back good, check continuity between alternator and ecm using a volt meter.
Also, the charging gauge is notoriously inaccurate on these jeeps. I would use a volt meter to check exactly what the charging voltage is when the car is on. There is a possibility that your gauge is no longer working.