Steering wheel locks while doing a backing maneuver

It doesnt lock... You reached the end of the turninh radius. You can only turn the wheel so far...

 

Something is binding because it is bent possibly. Tire in a hole or against a curb?

 

Do you think it might do that going forward? Find the problem now.

 

What truck are you driving?. I had a '16 Freightliner Cascadia that developed a steering problem almost exactly as you are describing. It would lock up when turning especially when at slow speeds such as when backing and when loaded heavy. I wrote it up many times, the shop would change the fluid, bleed the pump grease kingpins etc but nothing worked. They insisted the PS pump was fine and the steering box turned smoothly.

I turned in the truck with the issue never being resolved. I made sure to red tag it and told the fleet manager personally that the truck should not be issued until he test drove loaded and was satisfied.

 

Have you checked your power steering fluid?

 

Try to recreate it in a safe spot. Try holding the brakes to the floor and turn the wheel; I doubt the brakes are the cause.