Transmission problem 1998 Dodge Grand Caravan

The service engine light is on. Sometimes all the dash lights go out when I start the van and other times they do not. My reverse does not work when I put it in sometimes it tries and makes a grinding noise and other times when I put it in reverse it does nothing. What is my courses of action. What code should I be looking for. DO I NEED A WHOLE NEW TRANNY OR COULD I TRY SOMETHING ELSE FIRST cause the minute I walk in a TRANNY SHOP THEY WILL TELL ME I NEED ONE.