I have a 1997 12v all my indicator light come on and flash along with my voltage gauge showing not charging and all this happens only when truck is moving forward with engine under load or just rolling forward does not do it in reverse or setting at an idol not moving. I had alternator check and its good.

 

check the grounds and wire connections. could be a loose wire hanging and grounding when moving foward. could be a wire being pinched when torque of the motor moves too. my buddies truck ran perfect till he gave it gas then it would misfire like crazy, we found out it was a spark plug wire being pinched by the motor because of a bad motor mount. i know yours is a diesel but check all your electrical wires and grounds.