First, I am not a Jeep Electrician and really don't know much about Jeep and its electrical systems BUT...
Your problem sounds like a grounding or connector issue.
Have the alternator Load-Tested to see if it is holding up to the system load, clean the post on the battery, use some non-corrosive conducting paste to help keep things clean and the electricity flowing; use some on the ground connection too but not too much.
Then double check grounds, battery first. Make sure the “battery to block” is a good connection and the “engine to body strap” is still there and not corroded.
Under the Dash look for a ground strap for the gauges, maybe one maybe more, each gague may be grounded either w/ strap or through the meter itself to the dash panel, cehck for corrosion. Look to see where the feed for the volytagemeter comes from; I would think the wire comes from the ignition or fuse panel, but I've been wrong before.
An in-dash voltmeter only read system voltage not what is or isn’t coming out of the alternator, so if there is a problem this meter may only read what is making it to the meter.
Now, any Jeep mechanics around...
Sorry I can’t tell you exactly where the meter wiring taps into the main harness but get an electrical schematic for your vehicle and start tracing wires.