Slerp Z and X axis only

fromRotation = transform.rotation;
toRotation = Quaternion.FromToRotation(Vector3.up,((hoverGroundHit[0] + hoverGroundHit[1]) / 2));
transform.rotation = Quaternion.Slerp(fromRotation, toRotation, groundRotationSpeed * Time.deltaTime);

How would I make the Slerp rotation only apply to the X and Z axis?

You can use Quaternion.Euler to get your rotation in Euler angles representation. And then do something like this:

fromRotation = transform.rotation;
toRotation = Quaternion.FromToRotation(Vector3.up,((hoverGroundHit[0] = hoverGroundHit[1]) / 2));

OriginalY = toRotation.eulerAngles.y; // Save the Y
Vector3 finalRotation = Quaternion.Slerp(fromRotation, toRotation, groundRotationSpeed * Time.deltaTime).eulerAngles;

finalRotation.y = OriginalY; // Set the Y back
transform.rotation = Quaternion.Euler(finalRotation);