lost overdrive at 40 MPH
 

I have an '04 Expy with 5.4 4x4. I was driving home from work on the slow lane going 55 on flat highway when the engine rpms went up and the truck started slowing down. I pulled over and checked the transmission fluid, its full, and reddish and clear. no burnt smell. Also inspected around the vehicle and noticed no drips or leaks. So I pulled out into traffic again and the truck was fine until I reach 40mph and it did it again. So, I turned off the OD and the truck works fine. Turn it back on, and 40 MPH is my max speed.
What could cause this? Is transmission done? Or a simple sensor or something?
thanks for help

if it has no 4th gear then the e-clip came off of the od servo and is caught in the 3/4 shift valve. needs overhual. minimal parts. if it has no 3rd or 4th then the direct clutch is burnt up. not too bad about $2100.00.