I have not experienced this problem myself.
I have put around three hundred thousand miles on two of these transmissions. Of course I have had them since the, near new times!

If you change the oil in them at every thirty thousand mile interval as the books say to do, it does help I'm sure! Even a little sooner will even be better. Check the level half way in between as long as you do not see any leakage.

I have a '78 year that does not have the fourth gear drop out feature of dropping out for the overdrive.
There is a big ole' precautionary statement, in the owners manual, about backing it up while the overdrive is still engaged.

One would think, that this is hard at times to happen but it is really not.
You can shift it down,after you have coasted to a slower speed and then start shifting down not noticing any racing of the engine in the process.

You go ahead and get where you are going and park.
When you come back to the car is sitting there, in overdrive all set up. Or as the manuals says ... "Locked in overdrive."
The overdrive can bind up when it gets reversed or turned backwards and puts an undo strain on the internals. All in the manuals.
This might have started your issues on this transmission depending on how often, or far and how hard it was backed up. Especially in any hill country.

In the later years, definitely by 1984, the fourth gear and relay automatic drop out feature came into being!

There are many possible scenarios for a transmission tohave happen to it.

It does not have this feature or could be not working and was it backed up!

A bad fourth gear drop out switch.

It is an earlier transmission that was put into this car and has had enough with the back UP's!

The oil change interval was ignored completely and the oil level got really low at one time the overdrive quit working.

Did this ever happen since you owned this car?
Maybe some history details will help pin point what damage has been done.?

Trust me 30,000 is the appropriate time for the oil change. Type F fluid is good but it does have it limitations.

Phil