Can I move a ready or retreat a ready unit while in a showdown and I am defending?

To answer your question, we must look at the restrictions on unit movement during a showdown.

1. Can you move a unit (Standard Move) during a showdown?

No. According to Rule 144.1.c, a Standard Move "cannot be performed during a Showdown or Combat."

The readiness of the unit does not change this restriction. Even if your unit is ready, you cannot perform a standard move to move it from your base to the battlefield, or from the battlefield to your base, once a showdown has started.

2. Can you retreat a unit during a showdown?

"Retreating" generally refers to moving a unit from a battlefield to your base.

• Standard Move: As stated above, you cannot use a standard move to retreat a unit during a showdown.
• Ability/Effect: You can move units out of a battlefield during a showdown if a card effect specifically allows it (e.g., an ability that says "return a unit to your hand/base" or "retreat").
• Timing Restriction (The "Lethal Damage" Rule): Per the RiftJudge FAQ (ID 156), you cannot retreat a unit after damage has been assigned in a showdown. Once damage assignment begins, the damage is dealt simultaneously and lethally damaged units are removed during the combat cleanup phase, before you would have the priority to play an effect to retreat them.

Summary

• Standard Move: Prohibited entirely during a showdown, regardless of whether the unit is ready or exhausted.
• Retreat via Ability: You can use an ability to move a unit away during a showdown only if you use that effect before damage is assigned in the showdown. Once damage is assigned, the unit is locked in until the cleanup phase.

This is my interpretation based on Rule 144.1.c and RiftJudge FAQ (ID 156). No official FAQ exists for this specific phrasing, but the rules are clear regarding the prohibition of standard moves during a showdown.