Got it, I'll try again with more rust penetrant, unfortunately the nut is directly under one of the intake manifold plumes and I can only get a wrench on it, no breaker bar.

I did notice that the freeplay is in the coupler. I could move it back and forth by hand while under the car with absolutely nothing happening internal to the box. Is that the normal freeplay?

Yes. The steering shaft acts on the worm gear. The pitman arm is attached to the sector gear. The adjustment nut acts on the sector gear. No matter how tight it gets, it will have no effect on the slop present on the input shaft/worm gear.

The adjustment to the steering box is to help preload the sector gear to prevent excessive wandering and weaving due to the box being too loose and input slop from the steering wheel. When the box is adjusted too tight, you wind up with a different kind of wander/weave due to the box just staying wherever you turned it instead of self-centering and following the road contour. As I said, there's a fine line between too loose and too tight. Too loose is definitely better and safer than too tight.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Awesome info, just noticed you're in hill country, I'm in Pflugerville, small world!

As usual Diseasel has given us a lot of detail to consider. I have two things to mention about it.

I disagree that 1" is the best you can do on free play at the steering wheel. You can pretty much get it to zero without binding if you are careful on the adjustment in my experience. I have done this successfully with boxes with over 300K on them.

Also I believe the OP mentioned finding play in the coupler...to me that means play between the steering shaft and the connection at the steering gear, which would indicate a worn or possibly loose coupler.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I don't believe the play is in the coupler, just at the coupling. I can move the coupling and shaft without anything happening within the box.

Grabbing a 19mm crowfoot this afternoon and trying the lock nut again.

Can anyone think of any causes for non-centering that wouldn't involve the steering box? I know lower ball joints are usually to blame but those were replaced with my LCAs. I don't mind spending the money on a rebuilt gear box, if that would solve the issue.

I'll be impressed if you can loosen it with a crow foot. l believe my Indie did it from below with a box end and heat.

as for the play, if it is not loose between the shaft and coupler and it is as you say at the coupler then it must be play in the connection of the coupler to the steering box.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I'm going to try, if I can't I'll probably grab a coswroth box and have that rebuilt.