So, I bought this car used about a month ago - 1992 GXE with 170000km on it. Major vibration in steering wheel when braking, so I did the front rotors/pads last night (and replaced a spring/strut assembly). The rotors didn't look too old, so I was curious as to whey they would warp so badly. I was doing the rear rotors as well and there I found my answer. All four caliper pins (both sides) in the rear were seized in the torque member. What a PITA. No wonder the surface rust wasn't wearing off the old rotor. It just goes to show how important regular maintainance of your braking system is. If the previous owner had done this, the caliper pins would have been lubed regularly and my car wouldn't be on jack stands in the garage while I take the torque members to Canadian Tire to have them torched to remove the pins. So, my front rotors warped because they were doing almost all of the braking, while the rears were barely clamping on the disc.

 

I had the exact same problem on my old 92 maxima. The pins were rusted right in there. I spent over 3 hours trying to get one of them out and ended up bending it. They arent the easiest pieces to find either, I ended up paying $30 for one of them.