2003 Ford Explorer O/D Light flashes/ Check Transmission Li

Hello:

I took my car to the dealership for an oil change two weeks ago. A couple of days later the O/D Off light is flashing in my dash board and a prompt that says "Check transmission". I took my car back to the dealership to verify if the mechanic mistakenly did something to it to cause it to do this but they weren't that helpful. They insisted that since I only requested an oil change that they didn't bother anything else on my car. He then advised that I get a diagnostic check for 90 bucks. I did'nt think there was anything wrong with my car so I took it to BRAKE-O for a diagnostic test for $35 just to see without spending too much money. Well, this mechanic stated that there were no codes coming up on his machine. He took my car for a test drive and told me that my gears are shifting fine and it may just be a sensor problem because the light is flsahing and not sitting still on the dash board. My father reset my car to see if this stop the light from flashing but after driving about 10-15 miles. The light started flashing again. One thing I did notice is that since this light has started flashing, the time at which it starts flashing has increased. Where as it use to flash within in 5 miles. More like 10 -15 miles now. Please help me with this. I have only had this car for a little less than 2 years. I got it from Carmax around June of 05'. It is a pre-owned vehical but I refuse to believe it is the transmission because the car drives absolutely fine. I don't understand what the problem is.