As you said yours is a 2008 I am pretty sure it is out of warranty. Glow plug replacement is a 1 to 2 hour job based on where it is located and amount of time to take stuff off the engine to get to it.
I would see if you could find a local diesel mechanic who will have a really big scan tool that will detect if it is the plug, the wiring or something else and hopefully fix it the same day.
If you are blowing black smoke on startup, hard start, miss or noisy engine on start or all of these than it is a good idea to get it fixed before a long drive.
2017 SLX 195RB
2014 Jeep Grand Cherokee Overland, gas 3.7 V6
Andersen WDH hitch
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