My voice is "scratchy."  If I sing louder it is not as much of a problem.  I currently have severe post-nasal drip.  Could that cause the problem?

That could certainly be causing your scratchiness! Please see below for an article with advice from Dr. Reena Gupta (laryngologist): https://www.isingmag.com/allergies-voice/

Thanks...that was really a big help.  It doesn't make me sound any better right now, but it helps to learn that i'm not totally incompetent (well..not so far...)

So glad to help! The voice can be quite delicate, so small things can make a big impact.