97 won't go into overdrive

I have had many problems mentioned in posts: radio/reverse/interior lights issue, rpm's going up and down and up and down while driving freeway.

Replaced mlps seemed to fix all.

Latest issue is that two times the speedometer has stopped working, but working now. Major problem is that it won't shift into overdrive. Going on weeks now with this problem.

Any ideas?

Possibly the VSS (Vehicle Speed Sensor). This works the speedometer and tells the PCM when to allow shifts along with other variables.