02 f150 4x4 5.4L Automatic. Trans shifts fine, but once in OD for about 10-15 miles it will stop working. City drive or highway, its the same. I turn OD off and it drives fine. Fluids are good. Any ideas before i buy a new trans??

 

broken piece of snap ring stuck in top of valve body was a common issue, drop valvebody & inspect before replacing trans