A bent rack end will cause that, as it spins around it can play havok, but unlikey due to your diagnosis.
If you don't know how to diagnose problems its hard to rule anything out.
There are only a few links in the steering (5 if you count the rack bushes)
Jack it up, grab the wheel at 3 and 9 o clock. Keep steering the tyre back and forth by hand, If you feel any freeplay, your getting somewhere. Get a mate to do the same while you watch closley at each steering part to see whats got free play.
To me it sounds like the rack is stuffed, Ive never heard of the gears loosing a tooth, but it might be possible.
Could also be a caster rod bush/bearing gone, it might be binding the steering.
To test your rack bushes put it on the ground, get 5 fat mates to sit in the engine bay, then you get under the car and watch the rack while your girlfriend steers left to right throughout its whole range.
5mm of movement is normal, any more or abnormal movement is a bad sign, as under load with semis the problem compunds.
Really its impossible to diagnose over the internet.