Here's the solution (just in case someone else stumbles across this same problem and actually finds my question).
I'm an absolute beginner, so I'll describe this at a high level since I don't know all the correct terminology.
Open the ESX24 sampler on the Acoustic Guitar track. The selected instrument will be "Acoustic Guitar". Make a copy of that and rename it to Drop-D Acoustic Guitar. Bring up the instrument editor (the edit button).
Oddly, from the keyboard, D1 plays fine. But in my song, D1 is triggering two samples. When I play the note from the score (by clicking on the note), the instrument editor highlights to two samples, Zone #1271 and Zone #1399. 1399 is a chord and it is played in the range C1 to D1. I'm not sure how the ESX24 determines when to play each sample, since pressing D1 on the keyboard of the instrument editor only highlights 1271, but I don't really care. I changed the entry in the 1399 row to set the Key Range Hi to be C#1, so that it won't play on D1. There may have been another sample I had to do this to, but I can't seem to find it. In any case, you get the idea. Save the instrument, and now you can reliably play a drop-D guitar.
I hope that description was good enough to put you on the right track.