have you flushed the trans?
Is the fluid red? is it mitus fluid?
ive know thoses to have a torque conveter shutter but its more noticeable with light load and light on the gas
if it shifts fine i wouldnt replace it
anyhow i see a lot of noises and such when backing up to be rear brakes might be worth it to give them a look <img src="/forums/images/graemlins/kewl.gif" alt="" />