Check all of your wires to and from the battery, starter and alternator. They can look good from the outside but be corroded to nothing on the inside.

Are you measuring this voltage at the battery or at the alternator or is the PCM giving you this voltage?

Do you know that you battery is good? Any other possibility of a short?

If you leave the truck sit for a day or two does the battery go dead?

I kind of doubt that you would have two bad alternators myself. Most parts stores can test alternators on the vehicle, have you had them do this?

2000 Ext Cab SR5 4WD 2.7L 5-speed.