1999 Plymouth Grand Voyager SE Trans Problems

Can someone please give me some advice on what to check or what would cause the problem I am having. The van goes into reverse but does not want to go anywhere. It will eventually roll back on its own. When I press the gas it is like it is not in gear. But during idle with foot off the gas, it will roll back. When it was working it would only drive in reverse on flat ground. I couldn't even back up on ramps to get under it. The fluid was some low. Put some in it and it did not help. Please reply soon.