Is my shift solenoid bad, or does my trans need replacing?
 

The car used to take 10 minutes of warming up before it would shift into overdrive, now it appears to be stuck in overdrive and won't shift into any other gear but reverse and can't accelerate unless I neutral drop it because it's stuck in such a high gear. The fluid level is fine, I did check that.

It's a 1991 Toyota Camry 4 cylinder LE automatic 4 door.

Any ideas or help would be appreciated.