Emptied the power-steering fluid from the reservoir with a hand-pump siphon to replace the fluid with fresh, since the truck has 150K miles. The reservoir had been at the full mark. I have been getting just a trace of shudder occasionally during low-speed parking-lot maneuvering.
Ford calls for red Mercon V ATF as PS fluid. The unpleasant surprise is that instead of being red, what came out was dark gray. Maybe the Chrysler dealer that sold me the truck put in something else to top the fluid.
For now I filled the reservoir with Lucas power-steering "band-aid" type fluid just in case, and it's back at the full mark. After a few hundred miles or so, I'll empty the reservoir again and refill with clean fluid, then keep doing this until I'm happy with the color.