They don't get "shorter". I don't mean to offend if I read that wrong.
I assume you meant "shorted". They do "wear" electrically which is the reason they don't last forever. Your truck has about 120K right? If they're the stock glow plugs (didn't read about a replacement in your original post) then I would definitely just do them because they have 120K on them.
I NEVER tell someone to throw money at something, but glow plugs are really important to the starting and first couple minutes of drivability so it's not a bad idea to do them at 120K miles. You might consider replacing the small harness under each valve cover while you're there.
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Tuning, PCM flashing, and burning chips for 7.3s since 2008. Repairing all aspects of 7.3L Powerstrokes for 25 years.
Eight 7.3L PSDs in the driveway including a 1994 Crown Vic and 1973 F100/2002 F350. Looking for the next victim.
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