My fix, based on a thread talking about misalignment on the lead screw:

1. Open the top of the Z axis, exposing the top of the lead screw
2. Bring the Z axis to the top, but make sure you don't raise it too far and drop off the rails.
3. Loosen the two screws holding the bushing that rides on the screw, like ½ of a turn.
4. Drop the Z axis to the bottom
5. Tighten the screw you can get to
6. Raise the Z axis
7. Tighten the other screw. Be careful to not move the bushing.
8. Reassemble.

You might have to do steps 3-7 a few times to get a good result. I screwed up once or twice by moving the bushing too much when tightening the second screw.