You don't understand how the electrical system in your truck works, and you are very likely to damage something.
I will try to explain, but for now, trust me and everybody else here, and stop unhooking your negative cable with the engine running.
First of all, even a dead battery will supply a ground reference to the charging system. So unhooking the negative cable only determines that electricity needs a complete circuit to function, not that the alternator can run the truck by itself.
Unhooking a battery cable while the engine was running was fairly harmless in the days of point systems. Doing so often causes wide voltage swings and massive voltage spikes. But, there is not much on a point system that cares.
Modern systems use electronics that are sensitive to spikes. They are well shielded, which is why your truck will still run after you hook the cable back up. But there is no sense in testing the protection.
Modern alternators are capable of putting out 30 to 40 volts. They don't do so, because of the voltage regulator. The regulator measures the voltage from the output of the alternator to ground, and adjusts the voltage going to the armature accordingly to produce the correct output.
When you start removing grounds, you run the risk of removing the reference ground for the voltage regulator. If that happens, the regulator sees zero volts, so maxes out the armature to compensate.
In your case, you were fortunate enough that the battery ground was also the ground circuit for the whole system. The alternator voltage probably maxed out, but since the circuit was not complete, the voltage had no where to go.
Hopefully.
When you removed the ground reference, its possible that other components in the truck might supply a return circuit in ways that they were not designed if the alternator is grounded somewhere other than the battery ground.
When that happens, often the current flow is reversed through those parts. If its a headlight bulb, not much will go bad as long as the voltage stays in the right general range. If its an electronic system, usually it will be destroyed.
In addition to all that, when you disconnect or connect large loads suddenly, you will generate large voltage spikes in the conductors due to a property called inductance.
Say your alternator is cranking out 80 amps charging the battery, running the fans, lights, other stuff. You suddenly break that circuit while everything is connected. The current in the conductors goes from 80 amps to 0 in a fraction of a second. The magnetic fields around the conductors collapse back into them, causing what is called "Back EMF" which can result in voltage spikes in the opposite direction being generated in the hundreds of volts.
This is how a spark coil works. The points are closed, the current flows through the conductors wound around the primary coil creating a magnetic field that extends beyond the secondary coil. When the points open, the field collapses through the secondary coil resulting in several thousand volts output.
If you want to know if your alternator will run your truck with the battery dead, do it one of two ways.
Crank the truck, turn on every power using component in the truck wide open, and measure the voltage at the battery with a voltmeter. If the alternator can power everything and still maintain 14 to 14.5 volts, it should be good.
Otherwise, visit your local gas station or battery store and buy an old used battery. Put a quick charge on it, and see if it holds it. You don't want one with a shorted cell if you can help it.
Put the battery in the truck, run it down with the headlights, then jump it off and turn on everything to see if the alt can handle the load.
Either way, you really really need a battery in the circuit to keep things working right.
