Transmission fluid leak

You would have to drop the transmission pan to access the Value body and solenoids. Usually at this point on a transmission of your age, you would replace more than just a solenoid if you want it to be reliable.

It's most likely not the TCC solenoid. These transmissions are notorious for the splines breaking off the transmission input shaft. The shaft was not properly hardened at the factory. You will also lose overdrive when this happens. This is about a $1500 repair. Hope that helps! Jim