Are you sure its shifting into overdrive or is it the lock-up converter you feel ? Because it sure sounds like a lock-up converter shudder. How does the trans fluid look ? I would also have it checked for codes as some models can set codes for converter problems and not turn the ck eng lite on as that depends on the code sometimes. I would also see if there are any TSB's that pertain to it. I do remember we had that problem on front wheel drive cars as the trans control module can bring the converter clutch in on partial lock-up which is like slipping the clutch and if the converter clutch is not burnished in right it can give a shudder. Some of the car models had a TSB where it reprogramed the trans module so it turned off the partial lock-up and it only had full on or off lock-up. Ron