722.4 transmission now shifts from first to fourth

[mach4]

In the last couple of days, my transmission went from shifting really, really nice to where it will now only use first and fourth in the D position. If I floor it, it will not stay in first but shifts to 4th at like 15 mph. I can feel it trying to hunt for 2 and 3 but only goes to 4th. There is no downshift. It will go into 3rd manually and if starting in 1st manually will stay in 1st.

I've done extensive searching on this board and don't see these symptoms represented.

I've checked the VCV and that is fine. I've checked the modulator line and it holds vacuum fine. I've run it without the modulator vacuum line and there is no difference. The Bowden cable is working as it should. I don't have any vacuum leaks.

This is the 722.4 "one year" transmission that came on the '85 Cali 123 300D. It has about 220k miles and nothing is known about the history of the car. It's been in my 107 diesel conversion and has been running great for the past 7k miles. I've done nothing to the car that I can imagine creating the change in operation.

Any thoughts on the diagnosis or additional tests or things to check? Can I drive it this way while I explore options and fixes or is the next logical step catastrophic, stranding failure?