1996 Dodge Ram

I have a 96 dodge Ram 4x4 and I have appox. 120000 miles. I have had the truck for 10 years and I am experiencing my first problem. The Transmission after about 20 miles on the road kicks the overdrive off. If I shut down the ignition and restart, it resets and does it again shortly thereafter. It is becoming progressively worse. Yesterday, it was not shifting, I moved the trans shifter and it reengaged. But overall it shifts smoothly when in OD. It is a little stronger when out of OD. I'm thinkin a sensor on the trans, any idea's?
March 18, 2007.

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