Speed Sensitive Steering

[ClassicAU]

I have just found out that my power steering rack and pump have both kicked the bucket, as they are leaking a bit. I had a safety report done when I had the car lowered the other day, and they said I would be lucky to have a few months left in them.
Anyway, I was looking on ebay for the parts, as the bloke at the tyre place gave me a price that seemed overly high. I have come across two different types of rack, normal and speed sensitive. What is the difference between the two? Is this something the higher series AU's had as an option? If so, can it be adapted to a low series AU?
I did i seach and didnt really come up with much. My guess is that it has something to do with speed and feel, not too sure.

Evan