I have an 89 b250 whose overdrive recently started operating at different speeds than normal, and kicking in and out at 35 to 40MPH.
It is now operating as before, after resetting the ECM, but unplugging the battery for a while.
It is also possible my cleaning the base of the shift lever and electrical connector with wirebrushes and compressed air had some effect. I had reviously cleaned the connectors pins and sockets with Deoxit D5 with mini bottle brushes and precision swabs so I do not believe the compressed air did anything for the connector, but perhaps.
Or it could have been since I bought new solenoids, gasket and filter and 6 Quarts of atf+4 and had it all ready to drop the pan, that it decided to operate properly again.
If you do inspect the OD off button, be careful. I can't remember specifically what happened but I remember I was very relieved when I got it back together, installed, and it worked.
But Start with an ECM reset. I just unhook the battery for a while but some say to unhook it and turn key to start for 30 seconds. perhaps that is later models.