Chances are the alternator is the cause of your concern. The vehicle needs to be tested when the problem is occuring to properly diagnose it. If it's being tested when the problem isn't happening, everything will test good. Kind of like going to the doctor today, but you were sick last week.
What's probably happening is as the truck is driven and more heat is generating, a component within the alternator is giving up and unable to perform. Allowed to cool, it returns to normal operation.
Your answer is logical to me, however, it doesn't explain one problem (that I failed to mention) - the check charging system indicator comes on even when the engine is cold, after the first acceleration or at higher RPM. If the condition were caused by heat, how could it occur when the engine cold?
A bit more background: When this problem first began, it only occured under heavy acceleration. A week or later, it began to occur during every acceleration, and only stops when the engine RPM is steady at idle or at constant speed. At cruising speed of 45 - 50 the indicator is normal. At 60-70, at slightly higher RPM, the condition returns.
it still sounds like an alternator. Possible worn brushes, a bad stator or rotor assembly. It may be normal for the charging system to stop working under hard acceleration if the alsternator is controlled by the PCM, but again, it is still sounding as if the alternator is faulty. It will need to be tested when the condition is occuring to verify the problem, though.