| No. First, the only reason to loosen a truss rod when shipping an instrument is to prevent impact damage from spreading due to string tension. It was advice given for acoustic guitars decades ago. Has nothing to do with temperature nor humidity. It was if the airline cracked the top, the string tension might cause more damage. But if you loosen the strings for any extended period, then you MUST loosen the truss rod too. The rod is there to balance the strings so loosening one means you need to loosen the other.
For a drive as short as that, I wouldn't do anything except make sure the gig bags are protected from other stuff in the trunk smashing the basses. And you may need to have some set-up work done after the basses acclimate to your new digs.
jte
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JTE
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