What causes bad vibration when accelerating to 30 to 40. Whole car shakes.

What is your question?

The car has automatic transmission with 88,000 miles. When the car accelerates to 30 to 40 miles and hour the car starts shuddering or vibrating. If I let off the gas it stops. I thought I had bad tires so I changed them. I had the transmission flushed yesterday but still doing it. When I bought the car 11 months ago the fluid was brown but wasn't having any problems then. Sometimes when putting in reverse or drive I hear a clanking noise but most of the time its real smooth. What do you think I should do? LDJ18