This really sounds like a transmission issue so I think you are on the right path. You are talking about an Automatic transmission correct? You shouldn't have to put it in a lower gear to get it to work. The nice thing is these days you can get a rebuilt transmission for a great price.
First thing I would do is do not drive the vehicle far or fast. Ask friends and family for a transmission shop they trust. If you continue to use your work around you may end up damaging the transmission beyond repair.
A diagnosis will help you understand what has gone wrong but you will have to leave your vehicle at the shop for a day or two and they will charge you to tell you what is wrong. You could also phone an auto wrecker and you might know how much a used or rebuilt transmission is.
Hope this helps