It's hard to say at over 100,000 miles but it could just be a sensor, they do go bad and those overdrive transaxles lasted alot longer, why not put an electronic scanner tool on the Chrysler connector next to the fuse panel, it should have 5 wires and be blue, it mounts on the fuse box inside, with the key on the scanner will show any fault codes either the engine, transmission, or body computers have stored, after you replace any bad part that shows up, you'll need it to reset the TCM also. Good luck. The scanner reads each from that one connector so you'll have to chose transmission from the opening menu.
It can be used to moniter the electronics on the trans while your driving it. If the input and output speeds disagree speed and rpm wise it kicks right down into 2nd. And stays there until it is restarted. If there is a huge difference in RPM's there may be a problem internally.