Mine was fixed by Ford. They installed the higher output Ford Explorer (4dr version) pump on. No other goodies, just the pump.
What happens is the belt does not grab as well as it should due to how it is routed. As the belt ages this gets worse. The replacement pump requires less friction from the belt to produce higher output of the power steering pump.
So what Todd mentioned "probably" would fix it for two reasons. When your OEM Belt was new all was fine. As it aged you discovered the issue. So if you use a shorter belt the tensioner will put more pressure on the entire belt system. As a result, you will have better power steering performance.
Now, if you want to see something scary. Be going down the free way and floor it so it goes into passing gear. Right after that do a nice easy lane change. If your belt is ages and you have the OEM pump you will more than likely have ZERO power steering while attempting this lane change. It kind of freaks you out when your use to having steering and suddenly it is HARD to turn the wheel.
This is another reason to use a smaller belt. The routing of the belt tends to "Throw" the belt over the pulley of the pump as opposed to pull it around the pump.
It is termed "Belt Looping".
