Steering Locks up in 92 previa van

I am posting this problem that my friend is having with his 1992 toyota previa van. The steering locks up when it is driven hard... Anyone know any ideas on what may be causing this. If you need any more info just let me know and thank you for your time!

I had the same problem in my '91. It was the steering rack. Expensive if you get a dealer replacement part. I ordered a remanufacured and replaced it myself. Around $200-$250 if I remember. Should have it taken care of as it will only get worse and it can get dangerous if it locks at a bad time.

I just noticed the same problem on mine (1991) But I am having a squeel that appears to be coming from the belt. I was guessing that it is the pwr steering pump but now I am starting to wonder if the rack is somehow causing it?