I know I'm resurrecting a nearly decade-old thread, but just wanted to share my experience from yesterday.
I have a 2006 4Runner V6 SR5.
I was about a mile from my house (thankfully) and was making a turn onto a road when in the middle of the turn I felt the steering become very stiff making it very hard to turn the steering wheel. Within about 20 seconds there was some kind of oil/chemical smell coming from underneath the truck.
I pulled over and looked behind the truck to see if I could see any fluid/oil on the road but I didn't see anything. I continued to drive home, but the handful of turns were extremely difficult to make. Finally got the truck in my driveway, popped the hood and saw fluid everywhere near the lower left side (as I'm facing the front of the truck) of the radiator.
Like the OP stated above, when simply viewed from the outside the power steering fluid reservoir appears full, but when I removed the cap and looked inside, sure enough it was empty. The inside of the reservoir is dirty such that it appears full when viewing it from the outside. This is very deceptive. Now, there was obviously fluid in there minutes before, since I now have a lot of fluid leaking all over the inside of the engine bay onto my driveway. I don't know if a line burst or something like a pump failed, but prior to this happening, the power steering worked completely normal. No noises or whining.
I called my local mechanic and explained to him what happened. I told him there was no way I could drive the truck to his shop (only about 4 miles away) with the steering wheel being so difficult to turn. He suggested that I pour a bottle of power steering fluid (in this case, my truck calls for ATF) into the reservoir and see if that's enough for me to get to his shop before it all leaks out again. Otherwise I'll have to have the truck towed to his shop.
Either way, I'm taking the truck to his shop on Friday and will post back once I find out the problem.