2005 z71 steering whine
 

steering whine at low RPM. stiff turning left. pump replaced by dealership. problem still persists. gear box next?

I just put some Marvels Mystery oil in with my power steering fluid and mine is working fine now and has been for the last 8 months or so.

That's good news the dealership did it. They need to make it right.
Usually a whining power steering system in a Chev or GM is a sign of a once dry pump. Chrysler PS pumps are much more forgiving. You can run a Dodge pump dry (if you don't over heat it) you can top it up and your good to go.
Chev.. Not so forgiving.

powersteering whine
 

I added Lucus brand power steering additive instead of Marval. no improvment

Is your fluid foamy?

not foamy. I did however add Marvel Mystery yesterday. We'll see how that goes.