Quaternion/World Local rotation problem

I've spent the past few days trying to fix this by myself with no result.

I have a turret in a tank transform. I want the turret to look at something on the y axis, and not move on the x,z axis.

Here are some lines I've already tried:

 Quaternion.AngleAxis(targetQuaternion.y, Vector3.up);   //couldn't align with the rest of the tank
 .rotation = Quaternion.Euler(offset, targetQuaternion.y + offset, 0);   //didn't follow x,z correctly
 .localRotation = Quaternion.Euler(offset, targetQuaternion.y + offset, 0);    //didn't follow y correctly

 //Futile switching between Quaternions and Vector3
 Quaternion haggis = transform.rotation;
 Vector3 fuckwad = haggis.eulerAngles;
 Quaternion test = Quaternion.AngleAxis (targetQuaternion.y, Vector3.up);
 Vector3 rotten = test.eulerAngles;
 rotten = rotten + fuckwad + new Vector3 (-90, 0, 0);
 Vector3 rot3 = new Vector3 (rotten.x - 90, targetQuaternion.y + 180, rotten.z);

I believe most of my frustration is trying to find a way to set the y value globally and x,z value locally all in one go. I know in Java, I could've coded something like this, but what is the C# equivalent?

 transform.rotation.y=insertAngleHere;
 transform.rotation.x=0.0;
 transform.rotation.z=0.0;