Inspect the condition of the transmission fluid. Look for burn brown fluid.
The 96 MY 4R70W is a good transmission however, the 96 MY is prone to torque converter "shudder" under light load. The problem can sometimes be fixed by converting from Mecron IV to Mecron V. Note, the entire volume of fluid has to be changed for this fix to be effective.
When driving the car, have one run constant easy throttle application. Pay attention to the 3-4 shift and the TC lock up shortly afterwards. If the shudder is present, it will occur then.
More than likely, the shudder will not be noticable under med/heavy throttle.
In most cases, the shudder doesn't hurt anything but it can be a bargaining chip.