Transmission ... only works when I turn car off and on

Hey everyone. I had a super awesome time driving my Dodge Caravan 20 feet at a time! Ha.
OK. This is what’s going on. I have a 1996 Dodge Grand Caravan with … some mileage. About 180K and change.
Every two or three … four months since I have owned it (4 years 100K miles), maybe a bit more often, I have to top off the trans fluid. Never was a big inconvenience so blah. ANYWAY. Friday.
I pull out of a parking lot onto the road and my engine starts racing.
I tried to back up and it would ‘go’ into reverse (it’s an automatic) but not move. I turned the car off, checked my transmission fluid, topped it off, turned the van back on and drove. About 30 feet.
Transmission started to slip again. Engine racing.
I pull over. Turn the van off and back on.
Can drive another 20 feet.
I can do this over and over again. It still won’t go into reverse, but I can drive forward in 20 feet intervals.
I keep coming up with internet advice = sensor or solenoid.
Garage said I need a new tranny.
I don’t think I DO.
Ah. My check engine light has been on for about 2 weeks, intermittently.
IDEARS?

To check the transmission fluid level the engine should be idling and up to temperature. If you check the fluid level with the engine off, you’ll get a false level reading.

Tester

Oh, hey. :smiley: Thanks. I know! I AM just so accustomed to having to add transmission fluid every few months, it was my first instinct. Usually it will have trouble moving after a stop or on a hill, I add a half a quart and it’s golden.