Why does my battery light flicker and my voltage gauge go high on my truck

I can't tell you "why" but I can tell you how to figure it out/prove why. Your voltage gage symptom appears to suggest the alternator is overcharging at times. Do you have a voltmeter that you can connect to the vehicle and still be able to see the readings? (You need long meter leads) If the alternator itself is overcharging because the internal regulator is failing then you should see the system voltage going over 15v when the charge light comes on. However if the alternator uses a voltage sense circuit where an external circuit has to bring a system voltage signal back to the alternator, there could be a voltage drop occurring in it and the alternators regulator might not be sensing the real output level. You need a schematic that shows all of the vehicles charging circuit and you need to test system voltages both when it is working correctly and compare them to when it acts up.

Don' over think this problem!! Check all the ground circuits that you can find, clean and tighten, if no luck there replace the alternator!!! Most likely the internal voltage regulator is failing.