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Okay, I got it figured out.
The brackets that hold the vertical guide rods don't seem to hold the bed level when the stepper motors are disabled, and the weight of the bed overcomes the static resistance of the stepper motor and it sags on one side. This is not a problem as long as the power is on and the motors are receiving holding current.

After loosening all the rod mounts and carefully retightening them, it was no better.

So, what I did was to put a 60 tooth GT2 pulley on each Z-axis actuator shaft above the flexible coupling and tied them together with a GT2 belt and idler to keep tension on the belt and keep both actuators in time. Since then, the bed has remained level and have only had to make minor tweeks to the leveling adjusters.