How to tell if it is the power steering pump or the steering box?

[TOMAN]

Based on cost and effort, I'd change the pump. I believe that the LML pumps have higher pressure and volume, but that's something that you would have to determine. Or maybe a PSC pump.

And take a read here https://www.duramaxforum.com/threads/my-baby-is-sick-hard-power-steering.144843/page-2 Remember that this is just one person's experience.

Put a LML pump on old red and yes it is higher pressure and volume more vains in the pump , working good with a Red head box new hoses and a hydro boost , had a lot of slop , it was the lower steering shaft , replaced with a Dorman and all is good , made a HUGE differnce

ACDelco GM Original Equipment 20756713 Power Steering Pump ,​

Bottom Dorman OE Solutions Steering Shaft - 425-131 , Upper​

GM Genuine Parts 19329330 Upper Intermediate Steering Shaft​