Transmission vibration

What is your question?

I just completed replacing the rear engine seal on my G20 with 195,000 miles. The seal was leaking and no transmission problems. When the car is in park, reverse or neutral it has a severe vibration, but when you put it in drive it is smooth until you brake and as you approach stopping the vibration returns until you come to a complete stop. I put everything back together and replaced the transmission filter and pan gasket and refilled it with DEXRON III. Can anyone help.