I'm not sure what it is you're seeing, you have battery voltage (engine OFF or ON?) at the Alt's sm terminals. BUT if you add the load of the testlight the voltage drops to "0". Does the light come on for a split-second and dim orange and then to nothing too?
Again, I never tried that, not sure why that would happen... But, of course, if the engine is running you should see more than battery voltage (after the GPs are off and the PCM activates the charging system).
If this happens with the engine OFF, maybe the voltage reg is protecting something? A diode in the circuit?
In either case, if it were me, I'd pull the Alt and take it to someplace with a tester that spins it up and loads it (like most of the Auto Parts chains).
Here's another link (it has block diagrams), but unfortunately it too is for an '02 (I have an '02 so these are the ones I save
). In the mid '01 run there was a electrical revision, I'm not sure how it effected the charging system, might not apply to your truck but have a look anyway...