Could you, instead of looking directly at the sphere, do a lookat towards the x and z position of the sphere, and the y position of your object?

Edit: You could possibly use the following C# script to look straight ahead, but towards target:

Vector3 targetPos = target.transform.position;
targetPos.y = transform.position.y;
transform.lookAt(objectPos);

Putting the configurable joint just makes things more complicated. Why don't you disable the lookAt as soon as you enter the sphere and solve the rest by just approaching a location which is at a point which you calculate like [sphere center on ground] + [y-value of pivot point of the moving unit]. You just keep moving the last short distance towards that point above ground and no turning downwards will occur.

You could look into UnitySteer and the simple MovesToPoint vehicle as an example. Sounds like part of the problem is that the target you're attempting to look at is at ground level, when you should instead be looking at a point that is, so to speak, at "eye-level" for the unit. It's likely all you need is simply adjusting the Y of the target.

Update: Just noticed that's what Kvante said, and upvoted. Leaving the post up as the steering reference is still relevant.