steering wheel extremely difficult to turn while driving or at a stop. on 1990 Ford F-250

At first steering would turn very easy one way and most difficult the other. Then it is extremely difficult to turn both directions when sitting on the ground or loaded, however. If you unload the front suspension and set on Jack stands by the frame, the steering then turns like butter both directions. P/s has been flushed fluid level is full and it appears to be flowing. I've seen where people have replaced e erything but the gear box and problem still exsists.

Asked by for the 1990 Ford F-250
a pressure test of the pump is the first thing that needs to be checked. if that is ok, then dirt may have got into the steering box and that will need to be replaced.
dirt is the biggest culrit in any system that has fluid driven parts.

does this have a hyrovac system?? that is, the brakes run off the power steering pump??

We have seen the control valve in the steering gear box cause this concern. We replace the unit.