Ok it is either a power steering pump or the steering speed sensor. Your truck has what is called varialbe assist steering. There is a sensor near the end of the column that senses vehicle speed and adjusts how much power assist is needed. When they begin to fail you get some strange problems. Like for one what you explained and other things like at highway speeds you go to turn and it takes a second for the steering to react and then it is like you oversteered even though you have not. Does this make sense? If it don't please just ask I will try to better explain it. Now I figure you know what the power steering pump looks like so I will not include a pic of that but here is what the steering sensor looks like.
You can buy these at you local parts stores. The Napa # is 620-2250. Let me know if you have anymore questions ok.
Saturday, November 1st, 2008 AT 9:31 AM