I have a 2005 F250 4x4 with 37,s on it & somewhere round 55-60 mph it goes to downshift and kicks into neutral. I let of the throttle and it kicks back into gear until it happens again. Lower speed is fine or higher is fine. Nut right where I want to cruise in traffic it does this. What wud this be?

 

First suggestion would be to have the vehicle checked for any trans codes.
There could be an electrical problem where a solenoid is dropping out possible pressure loss involving the trans valve body. Hopefully the check for trans codes will provide some direction, otherwise internal trans problem where removal may be required.
Fluid level and routine service have been done, right?