I'm gonna assume power steering pump on this one...

Problem is that I'm getting a MAJOR workout taking turns at low speed in my truck... (IE Parallel parking or turning toght corners in a parking lot.)

If I rev up the engine a little, I can make it easier on me.

On the highway, there is no problem. It's only at low speeds or while stopped.

The truck in question is a 1989 F350 Diesel.

There is plenty of fluid in the pump.

I occasionally hear the noise from it as if I've got the wheel turned to full lock, even though I don't. It's knida a whirring noise.

I really hope it's not my steering box or valve body.

 

Here`s what my book says. 1. Improper tyre pressure. 2. Loose pump drive belt. 3. Low fluid. 4. Bent or poorly lubed front end parts. 5. Improper front end alignment, especially caster. 6. Bind in steering column. 7. Air in hyd. system (unlikely). 8. Low output or leaks in system. 9. Obstruction in lines. 10. Pump valves sticking or out of adjustment. If everything else checks out I would say the pump.