Dodge: Charging problems - 89 Dakota, starter computer, regulator problem

Question
My truck is a 4WD 1989 3.9 size engine, with 142,000 miles. It will only charge the battery when it is sitting still, out of gear. If I drive to work, when I get there the battery is dead unless I let it run for 15-20 minutes. Then it is fine. Here is what I have tried so far. Different alternator, starter, computer (all from a truck like mine with half the mileage.)Also a brand new battery. Nothing has helped. I have heard that it could be the voltage regulator. Problem is - some sources say the regulator is in the computer and others say it is in the engine compartment. I can't find it anywhere in the engine compartment, so I am assuming it is in the computer. Do you know for sure where it is located? Obviously if it is in the computer, I either got a bad replacement computer or that is not the problem. Please any ideas what other things I can try? Thanks, Dave

Answer
I would recommend taking the alternator to autozone and let them check your alternator,i know you mentioned that you tried one off an exact model.Some have the regulator in the alternator.Not sure on this one but when you tell them the year model,make etc.they can tell you.I was going to say the regulator,so all you can do is have it checked.I see no reason it would only charge while sitting still.When the engine belts are turning this is what initiates the charging.Not disputing what you are saying.I just think you need to have the alternator off your truck checked.This is the easiest place to start diagnosing the problem.