it means you have to crack open the transmission and replace the 4th clutch, aka 4th gear

In 97-02 year 4t65e's have a weak 4th clutch. There are ways to help the issue, like changing the 4th gear parameters with a tuner. Get a hardened 4th from trannyman's website, i hear these 4th clutches can be replaced in the car, its just a pita.