There are, in fact, four different valves in the valve body that are involved in the control of the basic/working pressure. It very well may be that one or more of them is stuck or that a spring is broken. And yes, your symptoms can arise from that.
Much easier to remove and inspect the valve body than the whole box.

Thanks Frank, I'll have to do just that. Looking at my valve body diagram, I see what you mean. 4 different valves that appear to be related to working or basic pressure. I'll remove the VB soon and dig in. Would be a shame to replace whole transmission because of a broken spring.

Folks, very interesting development!
I pulled the valve body, split it, and found a potential issue with the "Basic Pressure Control Valve".

Here's a photo of the valve after I removed the spring:
https://ibb.co/PtwBjSc

Here's the location on a diagram:
https://ibb.co/B263VfZ

Ok stupid question... this thing is supposed to move right?! I assumed so, because it's a valve, but it's not moving AT ALL. I have gently pushed the "rim" visible in my first photo, as well as further inside and I cannot get any movement whatsoever.

Assuming I haven't thought of anything, is this my smoking gun? If this was jammed in such a way that it was venting working pressure, that would certainly do it..

I just want to make sure I'm thinking of everything here. If it is indeed stuck, I'm not sure how I would get it unstuck without marring anything.

TIA!

Thanks Bill, I certainly feel like a Gomer sometimes at 32. I only really picked up a wrench in my late 20s so I am still learning a lot.

I just happened to replace the carrier bearing on my other w123, because it was shot. The symptoms were certainly different than what I’m experiencing on this car. Further, when I had the driveshaft out I inspected the bearing closely and it looked very good. Mercedes part too, so I didn’t want to replace it.

I don’t think the transmission is the cause, because those guys are seasoned professionals and they tested the transmission before shipping to me.

I am thinking driveshaft. One question: if one of the flex discs isn’t completely seated into the yoke of the transmission, would that cause vibration? I recall seeing that one of my new discs didn’t seat completely flush, there was a slight gap. I didn’t think anything of it, because that gap is front/back and I didn’t think it would cause a driveshaft imbalance.

Bill, you said that 2 cycles per second is typical driveshaft frequency, how can that be? It’s certainly spinning much faster than that. Just curious what makes you say that.

Thanks!