Transmission seems to go in neutral when I stop or when starting

I have a 02 Oldsmobile bravada. It is an automatic transmission. When I start the car or come to a stop it will drop into neutral. If I drop it into 2nd it will go and then I can put it into drive and it drives fine. Someone told me it was probably some boot on the transmission but not sure what it would be called. Anyone help me determine what needs replaced?