2005 Dodge Neon tranny trouble PO700 code

Sorry for the lack of info on the question. I was running late for work. Maybe I can give more details and it will help.
2005 Neon SXT
50,049 mileage

The speedometer started jumping wildly from 30 to 70 up and down while on the interstate. I drove about 10 miles and when I got off the interstate the check engine light came on and the tranny locked into 1st. Even though it's an automatic, I tried manually shifting from 1st to drive but it didn't help. I don't know if this is enough to help figure out what it may be. We don't have a Dodge dealership within 50 miles of here. The code came from the onboard computer. The PO700 was the only code it showed. My husband said that maybe it was a speed sensor but our parts store shows nothing in their books. This si SO frustrating that a car with only 50k on it might have a major problem. Thanks for all the help guys. I recommend your site to everyone I know.