2003 Chevrolet Tahoe overcharging

Okay, how many volts is it producing? As far as the alt, the voltage regulator is internal and not serviceable. The regulator controls the field current of the rotor to limit system voltage. Overcharging, then, is caused by a faulty regulator. However, I need you to do a simple check.

With the engine off, open the hood and check the battery voltage. It should be slightly over 12v. Have a helper start the engine and recheck the battery voltage. It should be in the area of 14v.

Let me know what you find.

Have the charging system checked at an auto parts store which does these check for free. Sometimes the vehicles' gauge will read incorrectly.
However, if the vehicle gauge is correct the charging system check can help pinpoint the problem.