Voltage Regulator

Usually the VR is NOT the problem. The VR MUST accurately "see" battery voltage. Check the voltage between VR and battery. To do that "get to" (in the harness) as close electrically as you can to the blue wire going to the VR. Turn ignition to "run" with engine stopped. Measure between VR IGN source and battery POS terminal. Do not disconnect anything "backprobe" a connector to get into it. You do not want to see more than just a few tenths of one volt, say, less than .3V

Now check the ground. Get the truck running long enough to "normalize" the battery--warm and after starting, and charged. Run this next test first will accessories off, and again with lights, heater, etc powered on

Stab one meter probe into the mounting flange of the VR. Stab the other probe into the top of the battery NEG post. You should read almost nothing, zero would be perfect

Usual causes:

Most often, voltage drop in the harness suppling VR IGN (power) terminal
Less often, ground
Much less often, problems with the battery itself, or a bad VR. Most VRs fail rather than "go off" just a small amount