Outdoor thinking - I don't see any USB charging ports on the Bluetooth speaker (as big as it is and for purpose meant for you would think they would include charging ports for small devices in tent etc.).
Closest I could find is where the one Jeep developer quoted the bed is 5 ft. or 6 1/2 ft. with tailgate down- so that makes the tailgate height around 18" (give or take a little in that where hinges are). So sounds like 18"