1995 Pontiac Grand Prix trans

Staying in one gear is a default when there's a problem in the system. The Transmission Computer runs tests all the time, and when it finds a problem, it sets a diagnostic fault code to indicate the circuit or system that needs further diagnosis. Start by having those codes read. Many auto parts stores will read codes for you for free, by most of the time they can only read engine codes. You may need to visit a mechanic with a full scanner that can access the Transmission Computer.

I'm afraid I don't have an answer for that one. The best bet would be to stop in at the dealership and ask what it will take to address that. On the newer vehicles they have all kinds of weird things that happen and the fix is to install new software into the computers. That's a major drawback of using all the inappropriate technology for things that never needed computers before.