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I was going to suggest maybe the voltage regulator, but the book I have says your truck has an internal regulator in the alternator, and you said it tested good. That is unless you have the optional 165 amp alternator, which has the regulator on the back of it.
If your truck starts and runs good, I would guess you have a bad connection to the guage or a bad guage. Really, the charging system is pretty simple, as in not many components. I would agree with a bad ground connection.
Good luck and please update.
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1991 F350 dually w/ 7.3 deezal
2008 Mustang GT
1993 Rodeo LS w/ 3.2 v6, 2wd (daughters)(Now totalled) 
1991 Yamaha XT600
1993 Honda CR250R
Hard tellin', not knowin'
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