Low voltage

[Christopher Gharibo]

I have a '91 coupe. I got a low voltage warning as I am about to start the car but the car started. The voltage gauge went from 10 to 12v. 

As I am driving I noticed the lights are not that bright and the voltage gauge reads 10v and is dipping. I got another low voltage warning. Somehow I got home and started checking the car. 

Alternator belt is tight. Battery is two years old and has never died. I pulled the alternator voltage regulator and the brushes have 1/4" and 1/16" length left. Clearly unevenly worn. 

What should my next step be? I am thinking either swap the voltage regulator or the alternator.

Thank you in advance.