Interesting. Just looked at the actual gauge. Looks like a shunt style gauge. Sorry for my misunderstanding, as that I figured Dodge would of switched over to voltmeter in the D-Series since Dakotas from that era used them and because of the different alternator and extra electronics for the Throttle Body Injection. On the 1988 D-series I've looked at I don't see the externally mounted voltage regulator on the firewall. If your truck still has the factory Throttle Body Injection and everything has been left stock then, the computer should regulate the voltage which is why you're not seeing the voltage drops or fluctuations that you're seeing. I will tell on the first year

1987 Dakotas they still had the old style alternator and external voltage regulator especially being the only year for the carburetor on a 3.9L V6. In 1988, they went to Throttle Body Injection and a different style alternator with no more firewall mounted voltage regulator. Your charging numbers are still within range and fine.