See answers below. Thanks Jeff!

Next:

1. If you stand (on something) so that you can see the screw when someone
else pushes the button, is there any movement of the screw while it's
making that "noise"?
The screw moves as if to lower the door for just a fraction of a second then
reverses
for equally long. It's as though the sensors are saying something is
blocking them. I've
jumpered the sensors and it still does it. It does this whether the
traveler is engaged or not.

2. Did the greasing possibly free things up enought so that the door moved
a bit further up than it used to and let the traveler jam up against
whatever it might hit at the back end of the track? Can you wiggle it a
bit in its present location, or does it seem "stuck"?

That doesn't seem to be the case. I can easily move the door (disengaged)
and with the
traveler in the down position (door is down), it's not anywhere near the
opener yet I still get
the same result. I unplugged the unit and checked the rear travel switch
(up limit switch) and all seems fine there
(ie. makes contact when closed, open when not closed).
At this point, I'm beginning to think a new unit is in order. Any
recommendations? Was looking
at the Genie Excelerator series for a start.

Thanks Jeff for the advice/troubleshooting help!
Cheers,
cc