HELP!!! 83 240D automatic tranny feels like a standard

If anyone can help me...my 83 240 D is an automatic tranny with 121K and it won't shift unless I let off the gas each time. It is worse when I tromp on it, but is still bad when easing into it. It feels like I have a standard, without a clutch. It has been suggested to me that it's a vacuum problem. I have NO IDEA about the vacuum system, and I don't want to get taken at the garage. I was thinking about replacing the tranny but don't want to if it can be fixed cheaper. I have already changed the filter and tranny fluid. Any help or suggestion would be appreciated.