1996 Plymouth Voyager Tranny stuck in first gear

Ok Let me explain what happend. yesterday my wife was driving the van and when she got home she told me that it was acting a little funny. So later on that night I took it for a spin and the tranny was slipping out of ger into netural and back in to gear with a jerk. so This morning I got some tranny fluid atf+4 because it was a court low and I filled it back up. it seemed to be shifting fine until I relised I was on the highway doing 65 at 8000 RPM's and right then and ther I knew the tranny was gone. Well I talked to the guy at the auto part store and he said it sounded like it was stuck in limp mode. so I changed the filter and fluid disconected the battery to reset the computer took it for a spin and still no luck. I need to know what this is. I can not afford a shop more less a new tranny regardless if its used or rebuilt I just dont have that kind of money. PLEASE HELP! I just need to know if I need to replace the tranny or if it might be something else.

thanks a million