Trans Overdrive Shift Issues

[Camalot10]

Ok, I have searched everywhere and cant find the answer to my issue. I have a 2001 Expo 4.6l 4r70w trans. It shifts fine in every gear. However, when in overdrive it constantly wants to down shift. It only does this when the trans has warmed up. When driving at about 50 mph it will all of a sudden start fluctuating from 2000 to 1200 rpm in and out of shift modes. It never slips but just kicks in and out of the two gears like it cant decide what gear it wants to be in. If I turn off the overdrive it will hold a constant 2000 rpm at 50 mph and shift through all the gears fine. Also my 4x4 light is flashing but Im sure that is a sensor issue since all my 4x4 functions work fine. Im hoping an easy fix but would like some guidance before I take it to the dealership Friday and have a scan done. Side note: I took it to auto zone to have it scanned but only came up with an O2 sensor malfunction. That would explain the check engine light that has been on since I bought it. Thank you.