Steering box/gear swap

My ta hoe is a Z71 so it is the off road package. When looking at them at GM parts online it appeared that it only called out the 2000 Z71 as being different. The part number for the one I found was Delphi re-manufactured listed as below:
Delphi 26058509 and GM original part number 88965789

1999 thru 2007 Chevy 1500 and GMC
Cadillac Escalade 2002 thru 2006
Chevy and GMC Tahoe Yukon 2000 thru 2006
Chevy Suburban 2000 thru 2006

When turning left it is hard when comming out of a parking lot and sometimes when driving and making a left hand turn. And it sounds like air is comming out but thats how it sounds. I lifted the truck up and everything is fine left or right as soon as the truck is lowered down its hard to turn left. I disconected the connector to the power steering pump somthing about the assist and no help. The power steering pump has fluid no leaks. Everything under the truck looks fine no play when driving straight it is fine no problems. Could this be the gear box? I thought if it was the gear box it would b hard to turn left when the truck is lifted off of its wheels but its not when lefted its fine?

Ok thanks! Oh and also when I am parked and I try to turn the wheels left as soon as the steering get realy hard to turn if I hold it in that position and I step on the gas it loosens up as soon as the rev's go down it seems to get stiff again? Would u be able to narrow that down for me? The belts seems to be tight enough. Thanks for ur input.

Oh ya forgot to say that before all of this happend for a while I had noticed that when I turn to the left at a full stop like when pulling out of a parking lot or a tight u turn I would fell like the front left tire was going over a bump? It seemed like something was rubbing making the tire feel like I was going over small bumps on the road but there was nothing there I though it was just the 22 inch rims I had on there but nothing rubs down there. But this has been happening for a while now but since I dont drive this truck very much and dont often make sharp left turns or turn the steering all the way. I got a new box and watch the video the truck is fixed thank you so much for the help.