91 Camry AT won't shift right

I have a 6 cylinder 91 Camry with automatic trans and 190k miles. Last week the trans began to act very strangely. The shifting became very loose in its feel and no longer worked correctly. In order to get into Reverse from Park, I had to shift to Neutral and then back to Reverse. In order to get into Drive, I had to shift past Drive into 2 or Low. The car seems to drive in 2 or Low like it were in Drive. However, I would like to find the problem. I have worked on this car for years, but I am not very knowledgeable about how the trans works, Suggestions please.