Encouraged by Jeffs diagrams, I yarded out the fridge and removed the panel + found the drain tube. It wasn't plugged on the bottom. It turned out that the hole at the top was completely plugged with ice.
so plugged that I didn't even know where it was (but since I knew where it was on the bottom, I could find its approximate location on the top). I waited 'til the fridge had finished a defrost cycle and then emptied the freezer and turned the hair dryer and some warm water on the area. it seems to be fine now.
of course you have to wonder why it filled up in the first place. I think we spilled an ice cube tray in the area and that might account for it. of course that may be just wistfull thinking.