well guys, I'm getting no SES light. There seems to be no problem with the engine itself, and the vibration is more in tune with the wheel speed than the engine speed. I probably should have said that in the first place, my bad.
I hadn't thought about the CV joint, but that would make sense. It could be held in it'* normal non-vibratory position when power is on, and then slack off and become out of balance when I release the gas. Hmmm... very interesting. I shall investigate this possibility. I assume that there is some inspection procedure in my haynes manual, in which case I will inspect them - on Friday.
Could be a wheel bearing, but there are none of the usual bearing noises to back up that theory. I know in my other cars with failed bearings, I could hear it before I could feel it. And, it didn't take as much speed to cause it.
Thanks for the ideas anyway. I just had to get a few second opinions, ya know?