Welcome to the AF!
My first thought is that you may need to flush your power steering system. You may have contaminated PS fluid.
Here is an example of how to do it:
http://www.toyotanation.com/forum/t19334.html
Although the 4Runner is set up differently, the procedure is similar. I just used a hand pump attached to an electric drill to empty the lines and PS pump reservoir.
All the above is probably wrong if you are getting the noise when going around a corner and no noise when you are turning the steering wheel. When you are able to turn the steering wheel silently and the truck makes noise while driving around the turn, then you are looking at a problem in the drive train.