I've got a 95 XLT 4.0 OHV 4x2, and the O/D light is blinking and it won't shift into overdrive. It didn't start flashing the O/D off light until one day when I was pulling a trailer with only a push lawn mower in it. (nothing heavy) I figured that connecting the trailer to the plug that supplies info to the trailer (brake lights, blinker, etc.) had caused the computer to get squirly since it had never towed anything in its entire life. (this was about a year ago) Also, I should note that I went up in tire size from stock size to the maximum size the truck could have,( without any lift or suspension modifications), without rubbing any while turning. I put on some expensive BF Goodrich A/T baja champion II to give it a much more pleasing aggressive look to it while also giving a slight but noticeable height increase. Didn't have any issues immediately after the swap with the tires. But no one at the Tire shop told me about the speedometer being off after such a modification. It didn't even cross my mind (I was about 19 with minimal experience with cars at the time) that the computer calculated speed by RPM of the wheels, and that it would be affected by an increase in tire size. Come, to find out my speedo was reading about 7 MPH too slow at 70 MPH (speedo would read 63 when I was actually doing 70) once I figured out this issue I looked into it and found that I had the model where I could simply change a small plastic gear at the end of the speedo cable that goes into the transfer case with a gear with one less tooth on it and it got my speedo back to being right on. I was hopeful that this would help my transmission issues since the computer would now know the actual speed I was going and would be able to correctly calculate shift points etc. But my hope was in vein. It still flashed and started to hard shift and still no overdrive. Even after unplugging the Battery letting the computer reset and trying again. The O/D light will not blink when I drive around town and it will shift smoothly with absolutely no signs of any trouble. But as soon as I get going about 45+ mph The light will start to blink and the issues will start again. I checked transmission fluid and it was not looking too good and it was low. So I added just a small amount until it was at the correct level while at operating temperature and in neutral. I have noticed that the bottom of the transmission there is a slick film over it that is apparently a leak of some sort but it must be a very minute one as the transmission fluid was not too much lower than it shouldv'e been. Seeing that the issue wasn't something that I believed was in my capability range of mechanical "know how" I took it to the most promising transmission shop in my area that wasn't a ford authorized dealership/mechanic. They have had it for 2 whole days and haven't even dropped the pan on the transmission. I was infuriated by this and they said it was because they cannot "communicate" with the truck (i.e. get it to give them any transmission codes, or any codes at all) but they said that if they couldn't get any by this afternoon that they would just tear into the transmission and try to figure out whats going on that way. Anyone here have any suggestions on why they can't read the tranny codes, or what could be wrong with the transmission, does it have to do with the larger tires, is this a common issue, i've got a million questions that I figured a TRANSMISSION SHOP would be able to answer but I haven't gotten a single one, very frustrating.