The pumps fail more often than the gearboxes so I would investigate the pump first. My first step would be to draw out as much fluid as possible and replace with synthetic PS fluid. Circulate the fluid and do it again. I have "repaired" a few Ford PS pumps by doing this. Make sure the belt is tight and it good shape. Double check your belt alignment too.
If not successful, put the front end on jackstands and listen as someone turns the steering wheel back and forth with the engine running in PARK.
1977 Cherokee S