The transmission control module. There’s a place I think in Hialeah, FLA has one somewhere around $250-300, just install it and go. But is the speedometer working, if not that coutbe the problem cause control module doesn’t know your speed. I helped a lady friend a few yrs ago, hers stopped shifting out of first gear. I checked everything. Installed the new module. That was it. Not sure if you have access to a snap on scanner but should be able to use it to tell transmission to shift, if you can then it’s the module. My opinion.