My 97 f250 has hard steerin when your just idling along through parking lots. It pretty much doesn't have any power steering at idle speed when your trying to park in parking lots. Makes it pretty much impossible to park in close quarters. Power steering pump is full of fluid and is not whining or leaking. It has new front axle u joints.

 

mine is same way but never tried to up the idle to see any difference

 

Collapsed high pressure line can cause this, also a bad power steering pump.

 

Do you have tilt steering? Also is there any noise while turning your steering wheel? If the answer to both is yes, you may have worn bearings in your in the upper steering shaft - it's a common problem with our vintage. I just had mine replaced - bearings and upper steering shaft. The broken bearings destroyed the shaft. I heard the noise and ignorantly ignored it until it started affecting the steering effort. Also, if you have tilt steering, don't use the wheel as a grab handle to pull you in the cab. That's what most attribute the wear to.