Right and left turns are fine at speed, but when slow or no speed, right turns are muscle exercises while left turns are still fine. I think it is the steering gear box. Anyone agree or know differently?

GM has issued a TSB on this. TSB #05-02-32-008B. If you have the 2500 or 3500 series, this is considered normal if all other PS components check out. If you have the 1500 series, you can replace the gearbox with the 680 to get better power assist when parked.

Also check out the warnings on this TSB

Power steering pump, power steering fluid, fluid flush (http://www.ricksfreeautorepairadvice.com/Diagnostics/BySymptom/stiffpowersteering.html)