My 2003 f250 is having tranmission problems!?

I have had problems in the past winter with my tran. shifting hard before it warmed up. Now the overdrive off light on the shifter keeps blinking randomly. It just started doing it after i changed the battery. The trans. seems to shift hard from 1-2 and 3-4 I'm not sure about with overdrive off but it seems to run pretty high in rpms out of overdrive. It's a 2003 f250, 4 speed trans, and 4x4. I think its the shift solenoid, if so how do i check it. its not showing any codes on my code reader. also where is it and how do I change it if it is the solenoid, or if its something else please describe in detail. When i turn my truck off and back on the overdrive lightdoesn'tt blink any more. At least not for a while. It has 107000 miles and Idon'tt think the trans. has been flushed.