Installed rack and power steering pump is good but

have you inspected the steering rack and pinion. they do wear out and need to be replaced.

Possible causes for a stiff steering wheel are a loose or damaged power steering belt, low power steeringfluid levels, a damaged or leaking power steering rack or a broken power steering pump.

What can cause a steering wheel to be hard to turn? ' Reference.com

https://www.reference.com/.../can-cause-steering-wheel-hard-turn-8b7a03d931c5d8cd steering wheel is hard to turn Google Search

sounds like the valves in the box or rack want renewing.can you get a recon rack?.or you could do a pressure check of the pump by teeing in a pressure guage

make sure all air is bleed out of the system by turning the wheel fully to one side then the other until the air is out of system and fluid level is at the correct level before replacing the solenoid.

this problem indicates a bad steering rack. the bushings in the rack are worn and have caused groves in the housing causing the binding. the only solution is to replace the steering rack. if a problem was the power steering pump it would not get easy when you turn the steering wheel further the problem would be there all the time.

This can be caused by many things.
A worn or slipping belt.
Low fluid in the power steering pump reservoir.
A bad rack and pinion.
Bad power steering pump.
Tire pressure low.
I did have a power steering pump go bad and cause this problem.
I was told that, no way a power steering pump could cause this, and after I had the rack replaced guess what, it was the power steering pump after all.

I'm going to agree with the rack conclusion. Keep in mind I can't see it for myself.
If you take the load off the front wheels, you should be able to turn the steering wheel without effort.
If the effort changes while turning the wheel, the only things affected would be the rack and the column.
If you notice it turns ok with the engine off but not with the engine running, it could be something with the pump or one of the lines, or still something with the rack.

If you know the pump is good and I don't see
how you know that ,then the steering rack is seizing

You could flush the fluid,add an inline filter,and some
lubgard steering additive or dealer steering additive
(not any Lucas Products)

I thing you need a new rack.

That is assuming the suspension parts are not tightening up

The trucks like yours have a rack and pinion steering, and when rack and pinion steering need replacing they get hard to turn in one direction just as you have described.

the problem with this hard steering, is the lower steering shaft to the second u joint. and it is a lot cheeper than a steering rack.