Trans wont stay in overdrive; it keeps shifting into and out of "OD".

What is your question?

99 Cummins 2500, with either the O/D on or off, shifts perfect till 4th. It shifts into 4, then immediately right back to 3, with any accelerator being used. Is there a "O/D" sensor?, or is this, the gov sensor?

How long has your 1999 Dodge Ram 2500 had this problem?

Within the last hour.