I finally got around to fixing this a few weeks ago...
I tested the water pressure at a hose-bib downstream from the PRV valve and found my water pressure was 118 PSI !!!!! And I verified that the contraption dripping water next to the PRV vale is an over-pressure valve. I cleaned off the face of the overpressure valve and found it is rated for 120 PSI - I guess that's why it's dripping - it is at the limit.
I picked up a single-union PRV valve from Home Depot - a Watts model for about $70. It was approximately the same size as the one that was installed. It took a little leverage to get the old PRV valve off, but it finally did come loose. The new valve installed without a hitch. When I turned the water back on, I was at 60 PSI and the over-pressure valve was no longer leaking (so I decided to leave it alone). The water pressure is noticably lower throughout the house. Showers no longer feel like you're being hit from above by a firehose (which I quite liked), but I'm sure the valves and other fixtures are thanking me for it.
Thanks all for your help and encouragement - a plumber wanted to charge me $275 to do the work.