need to have the truck running a fast idle w lights/heater on. It's a charging prob so need to duplicate the actual conditions where it's acting up. Bottom line is what charging voltage are you getting at the batt posts & not too much (overcharging) and not too little (eng wont crank). Assuming batt is good. Might make up some jumper wires w alligator clips on each end & jump each connection till you find the one that makes the charging voltage go back to normal ~13.8-14.2. Batt positive post to (1) blue wire alt male field terminal (2) top VR terminal in the triangle. (A) Green wire male alt field terminal to "side" terminal in VR triangle (B) alt case to batt negative terminal (C) v reg case to batt neg post (D) eng block to firewall (E) firewall to batt neg post. The black bulkhead connector is a frequent culprit and as said & from your posted readings the blue wire sense circuit is showing too low which is making the VR tell the alt to up the rate to fix it.