Did you test at idle? Or did you pull the alternator and have it tested? Could appear to be fine with no load on it.
I just had a bad voltage regulator in one of mine and it seemed perfect at idle but under a heavy load it would fluctuate enough to make the headlights and dash lights flicker
I would suggest checking and cleaning up all the grounds under the hood and 2-3 of them in the cab that are under the kick panels... one is near the e-brake pedal assembly and I believe there are two on the passenger side near the ECM.
Sorry for the delay, I have been working out of town trying to pay for this truck haha.
I replaced the alternator with a used unit and still the same situation arises. I have cleaned the grounds and all the fuse terminals look to be clean.
I appreciate all your help, honestly I'm getting to the point of cutting my losses and selling the truck.
The ignition control module is different for a manual transmission and an automatic. One has provision for push starting.
The module also controls amperage to the coil, giving more in START than in RUN. If the module is cooked, it might enable spark in START, and cut it off in RUN. The module produces a signal that the computer uses to know the engine is running in some years, and will enable the fuel pump again after the initial 3-5 second run to pressurize the system.
Be sure you have the correct module, and that your ignition switch on the column is in good shape. Mine seemed to disconnect power to the ignition when released from START, and the engine died intermittently...