PO700 code

I have a Chrysler 300m 1999 millage is at 150,000 engine size is a 3.5 liter V6. I have had no sings of any problem until the other day when I was driving down the freeway and noticed the rpm at 4 when only doing about 60 mph. I also noticed that the mph was not showing on the speedometer. I took it the a local repair shop where they did a scan and code PO700 came up. They where not sure what this code meant and how to repair the problem. I was wondering if you could tell me what the code means and what repair needs to be done. I looked it up in my book and know that it is a transmission problem but am not sure if it is a sensor or if it is a major repair problem?