That is hard to say as there are many things that could do that. It sounds like the input shaft has a bad gear or the planetary gears are stripped.
Take it to a tranmission shop for diagnosis as a regular repair shop is not equppied to work aon automatic tranmissions. It take a lot of expertise and special tools to work on them. AAMCO is over priced, so use the search engine on this site or the itnernet to find a transmission shop near you.