On Saturday, October 22, 2016 at 2:00:58 PM UTC-4, Oren wrote:
On Sat, 22 Oct 2016 10:46:53 -0700 (PDT), Chris Jenkins

Sorry, I think I explained poorly; the walls of the old shower were plumb, its all the rest of the bathroom that's not, so I'm trying to figure out the best way to compensate. Everything in the shower is square/plumb/level, it's where it meets the not-plumb walls of the room where the issue is. It looks to me like in the past the lower half of the walls were torn out to upgrade plumbing and electrical, and the drywall they replaced it with makes those new walls thicker than they used to be.

This is our eighth year in this house and the last room to be remodeled, and I swear something like this has come up during every single project we've tackled...

Here in the West, Homes with tall walls, there is a method to get the
wall studs plumb. Using levels both horizontal and vertical, studs are
shaved with a wood planer and shims used to correct them. Something
I'd never seen back East. It may be an option for you. Is the
drywall sheets the same thickness in you shower?

Shave & Shim would mean having to remove the present drywall, though.
Normally, here, this is done before drywall is hung.

YMMV

Studs both in the shower and the rest of room is level, the issue in that materials that make up the walls outside the shower (plaster on top, drywall in the bottom) are of differing thicknesses, so the walls are not plumb. Keeping everything plumb and level in the shower is taken care of, it's figuring out how to transition those new walls to the non-plumb old walls on either side