2000 Suburban steering problem

I have a 2000 Chevy Suburban 1500 with about 135,000 miles. When sitting still, on hard, dry pavement, or backing slowly out of a parking space, the steering wheel will not turn or is very balky, at best. Sometimes it will only turn about 1/2 turn each way before it hits what sounds like the normal end stops. There is no problem when rolling or on wet pavement. Is it the pump, the steering box, or do I even have a speed sensitive steering? Thanks for any help.