93 explorer wont shift into overdrive

1993 explorer 85,000 miles, shifts fine, but not into overdrive.
could it be a relay or solinoid?

1993 explorer 85,000 miles, shifts fine, but not into overdrive.
could it be a relay or solinoid?

Ask for an AL4D [transmission type] modulator.
Replace modulator and any related vacuum hoses.
Your transmission overdrive is not electronic.
There is an overdrive servo that could have a weak spring.
Though the spring failure is rare, it still is a posssible cause.
Good Luck