It's quite common for the voltage regulator section of the PCM (computer) to fail while the rest of the PCM is fine.
Rather than replace the $600 PCM just replace with a $20 regulator.
I am going to post the pics and parts I used for my external voltage regulator install on my '97 due to some confusion that I encountered when I was doing my own install. Hopefully this will help with the issues anyone has. PM me if you have any questions or problems. Bill, if you could move this to the tech section that would be great.
This is a picture showing where I mounted the voltage regulator, below the wiper panel on the passenger side of the truck. This works great because it's grounded and tucks up in there nicely. I used a Ford style external regulator from O'Reilly's, part number R400 cost $14.99. It can be seen here:
Item Detail | O'Reilly Auto Parts
Viewing from the top down, wiring connected to the regulator is: alternator field connected to upper post, switched power, alternator positive (output).
This shows the alternator connections. The first thing I did was unhook all the original electrical connections from the alternator. I then took a wood chisel to the little black molded box and broke it apart to reveal the individual ground and alternator output cables. Then, pull apart the wire loom that runs over the upper alternator mounting bracket that has the two wires connected to the original alternator field, and cut, heat shrink, and tape them. Re-install wire loom over those wires. Now, the 2 little green and orange wires need the ends cut off, heat shrinked, and taped. You can see them zip-tied to the alternator ground cable. The rest is easy, take the wire connected to the top post on your regulator and run that to the upper field connector. In my pictures that is the orange wire. The 2nd wire down on the regulator goes to switched power, I used the same lead that I power my pyrometer with. The final wire goes to the positive (ouput) wire on the alternator. Lastly, you need to ground the other (lower) field post. I looped a short wire from that post to the ground connector on the alternator case.