Doesn't want to turn left

[Nima]

Hi guys:
This is a 2005 chevy D-max. In stand still, I can move the steering wheel to right normally but I can not (I CAN NOT) turn it to the left!. If truck is moving I can turn it to left but not easy/smoothly. I flushed the P/S system. I jacked up the front and except for the tear in pitman arm rubber boot, I didn't see or feel any paly or slack. When the front was in the air, I didn't have any problem to move the steering wheel to left or right. is it just the pitman arm has failed? Any ideas? Could it be the steering box or something else?
I disconnected the steering stabilizer while truck was on the ground in hope that is the culprit, no luck. Still doesn't want to turn left. I lifted the front again and as soon as tires clear the ground, steering wheel is noraml? I don't see any binding, anything.
I guess if nobody has any ideas I have to start changing parts. My original plan was to change/upgrade the pitman/idler arms and put cognito braces on them. Now I am thinking to change the steering box since that could be the only/main cause. Is there anybody here has knowledge about the internal of steering gear boxes? How the internal work? Is there any left and right vlave or something which can fail seperately? Why is it haapen under the load only?
I know, long thread and too many questions. Any help would be appreciated.
Thanks