I sent this question into Verizon Wireless as well. Here is their reply:
Thank you for contacting Verizon Wireless through our website...I understand that you've been having an issue trying to decrypt files on your handset. At this time Motorola hasn't provided an update for your phone to be able to do this. Without that, we're unable to assist you because this is a phone issue and not a network issue.
So it looks like Ryan was correct in his comment that this is an issue with a Motorola OS add-on, not vanilla Android.
Since originally asking my question, I experimented some more with my phone. My phone has 2 check-boxes for setting encryption: one to encrypt the SD card (that I mentioned in my original question) and one to encrypt the main phone memory. When I unchecked both boxes, my phone decrypted all files, both on the phone and on the SD card. So, that is my solution for now.
Thank you Ryan for your comments -- they helped lead me to this answer.