01fordexpy, first off, put the battery on a charger and get it up to full charge. Now, using a multimeter, connect it across the battery set to the VDC scale. It should be reading 12.6 VDC. Start the truck (with the multimeter still attached) and see what the multimeter reads now. If you have all electrical stuff turned off, you should get 1 of 2 voltages, 13.7 VDC or 12.5 VDC. If you get 12.5 VDC, you have either a bad alternator (which the voltage regulator is part of the alternator) or the cable between the alternator and the starter solenoid is toast. When the cable is bad, you tend to see a voltage in the 12.6-13.0 range, but definitely not up in the 13.7 range. You can prove the cable by turning on the headlights, the A/C on high (front and rear), and anything else electrical you may want. If the voltage drops down to under 12.6 VDC, bad cable.
If the voltage at the battery with no electrical loads is 12.5 VDC (or lower), then your alternator is not outputting anything. First get the alternator checked to make sure that it is functional. If it is bad, install a new one. If the alternator is good, then odds are the tan/yellow wire between the starter solenoid and the alternator has an issue.
