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The alternator is designed to drop it's voltage under load, it says so in the owners manual. My Isuzu's have always done that. You should have had the guy replace the brushes while the alternator was out and you would have been set again for years.
Yup.
A simple test to see if it's working properly is to turn on the headlights. With the headlights on voltage should remain ~14V regardless of what you are doing with the throttle.

Steve

'97 Rodeo 5spd. 3.2L SOHC V6 4WD