1998 Plymouth Grand Voyager limp mode

My van is a 1998 Plymouth Grand Voyager with 118,000 miles. It has a 3.3L engine with an automatic 3 speed transmission. The problem does not create any strange smell or sound. I have a minor oil leak from the pan gasket - unrelated to current problem. The problem is as follows: in March of 2007 we had the transmission fluid changed (completely a maintenance item, no problem existed at the time). Two days later on a family trip while traveling on the highway we noticed the transmission go into limp mode (staying in second gear). Along with this was the "service engine soon" light. Have had the car looked at and no obvious cause and it appears that there are no mechanical problems. Recently within the last two months, this limp mode seems to be occuring more and more often. The limp mode condition is reset when the ignition is turned off and the car is restarted. Today, November 4th, had error codes read from computer and they are P0700 and P1776. I would like any suggestion on what could be causing the problem. I am considering 4 possibilities: 1. Wrong trasmission fluid (at service they put in dextron plus additive), 2. One or both of the speed sensors, 3. Transmission control module, 4. Solenoid assembly. I am open to any and all considerations, thank you.