This is important. Repair shops have accounts with auto parts stores. So they mainly use after market parts. The only time they normally go to the dealer is if the part is not available after market.
So with that being said, when they take the alternator off to lets say send it back. The Auto Parts store may have a bench test machine. So they mount it on the machine to see what it produces as far as a charge. Most of the time it will bench test fine, but as soon as you put a load on it ( meaning installed ) it can't produce a strong enough charge. So I can't tell you how many times I have seen bad alternator bench test ok, cause there is no load on it.
When you have a good working alternator, if you have the truck running and a volt meter on the battery with lights off and AC off, you should see a charging voltage of around 14.3 volts. It should not be much higher than that. If it's over 14.5 volts, then it's over charging.
Now, when you want to load test it, with the volt meter still on the battery and truck still idling, turn the head lights on, AC on and rear defroster. The charging voltage should be around 13.8 volts. No lower.
That Is a simple test when using a volt meter.
Which you can explain to the repair shop to have them quickly check this alternator for you