The bit you have bust is the steering box/panhard rod tie-arm or tie-bar. The original RRC didn't have this, it had the large casting in your picture which locates the panhard rod and therefore the axle sideways under the chassis. Mounted close by is the steering box. It was quickly found that the chasis twists even over this short distance, as you have observed, and steering control is affected.
The good news is that all coil sprung LR have one and they are probably all the same part. I suspect yours has either been fitted incorreclty or was somehow weakened or faulty from new.
I've probably got one kicking around if you can't find one anywhere else.