1999 Ford Explorer Question: O/D light flashing causeing my transmission to not always shift

When the O/D light is flashing, it is indicating that the transmission control module has detected a fault in the Transmission, or the electronic control system.
The next step is to retrieve the diagnostic trouble codes that are stored in the transmission control module and diagnose them accordingly. This may be best left to a transmission repair shop, I would look for an ATRA shop: http://bit.ly/ATRA_Info
If you do get the codes, post them here and we can give you some more info.
Good Luck!

I drive an 2002 Explorer with 168,000 miles; it has the 5R55W automatic transmission in it. Like many of you, my O/D light started flashing, and I was getting a hard shift from 2-3. After much research , including from this site, I learned that the problem is NOT solved by replacing the transmission. The problem is solved by replacing the auto trans solenoid pack (drop the pan, replace the solenoid pack, replace the the pan and refill with fluid). I had the repair done for just over $500 - labor & parts. The challenge is finding a transmisson specialist who understands the problem and is willing to do the solenoid replacement INSTEAD of replacing the transmission. My transmission is now shifting silky smooth.