I got a 1983 300SD Turbo Diesel with ~240k miles on it. Recently I have been having transmission issues.

About a month ago my transmission would not shift out of 1st (yes 1st, not the usual 2nd gear start). It would exceed 4000rpm @ 25mph or so yet not shift to 2nd. Occasionally it would jump to 2nd gear and I could get going a little bit faster. Got it home, drained the transmission (fluid dark) and torque converter, replaced the filter, gasket, and refilled. Worked fine until this week.

Last week/early this week, 2nd -> 3rd slipped some until the car warmed up some. Tried adding a bit of Lucas to see if that would help with the slip, no dice. Took it to a local lube place and had them flush the transmission. Came out a bit dark again (but not too bad). After that the slipping mostly went away and the car kept working fine.

Yesterday car is back to square one. Car would mostly work from 1st to 2nd gear. But refused to get out of 2nd gear, over 4000 rpm @ 45mph, but wouldn't shift. Decided to try draining and changing the fluid once again. Did a basic drain of the transmission (didn't include torque converter this time around since it can be a pain to line up that drain plug), fluid didn't seem bad.

I read that sometimes the Lucas additive can make the fluid a bit too thick and can prevent the pistons from moving, so tried running it without any additives (as it should be). No change whatsoever, 1st and 2nd work great, wouldn't shift to 3rd.

Tried the Trans-X stuff that some people here have shared some success with, since I'm at a wall I give it a whirl. This time I dropped the pan and changed the filter again. The filter I pulled out was somewhat degrading inside, luckily it didn't come apart and get inside the deeper parts of the transmission. Swapped the filter with a new one, and filled again. Tried with the Trans-X again.

During the test drive, same as usual, except it did shift to 3rd for ~25 seconds. Took 3rd up to 75mph, but then wouldn't shift to 4th at all (just like 2nd to 3rd issue). But once I went back to 2nd gear, I couldn't get back to 3rd again. So here I am now.

Various other info:
1. Bowden cable adjustment, didn't work.
2. Pulling line off modulator hoping for some hard shifts, didn't work.
3. Reverse and Drive (1st or 2nd gear start) work fine and engage quickly.

As long as it doesn't involve 3rd or 4th gear, everything seems peachy.

Possible issues?
K1 and K2? 2nd->3rd and 3rd->4th aren't slipping (since they don't work at all), but could it be these pistons are broken/stuck and refuse to work at all?
Honestly, apart from that I can't think of anything, drive engages and it certainly has the power so I doubt it's the B2 piston (which would be bad if there was no drive at all, correct?).

The transmission is a 722.3? I found the ATSG for the 722.3 and 722.4 and looked through it.

Anyone know the part numbers for the 722.3 for the K1 and K2 pistons? Or, is it the same as the 722.4 as seen here:
722.4 K2 Spring Part Number

I don't want to start ripping apart the valve body if I don't have to since it seems like a huge pain to deal with. Certainly would like some insight first.

So any thoughts to what this might be? Thank you immensely for your help.