Overdrive light flashing No lock up

try to turn the overdrive completely off then check if its still slipping if not its a simple fix if its slipping still its a big problem and take it to a transmission place.

There will be a code in memory which will indicate the problem. From you saying it flashes at cruise it is most likely your torque converter is not locking up as it should.

Roger, that doesn't even make a whole lot of sense to me. If the overdrive is causing the transmission to slip, wouldn't it also cause the fuel mileage to go down? Are you sure the transmission is slipping? Or is the transmission simply shifting in and out of overdrive? If it is, is it shifting at the proper speeds? Driving at around 40 - 50 Mph will sometimes make the transmission go in and out of overdrive repeatedly. This can be perfectly normal.

Have you checked your transmission fluid level?

Is your overdrive off light flashing? If the transmission is actually slipping the OD OFF light should be blinking at you to let you know there is a transmission problem. Either that, or the Check Engine light will be coming on. In either of these cases, you need to scan your onboard computer network for possible fault codes.

You should not use overdrive unless you are steadily going highway speeds. I had one of those and my ex liked to use cruise control on windy coastal roads. This caused it to shift back and forth from 3rd to overdrive constantly. So I disconnected the cruise control so he couldn't use it. Lol. I'm not suggesting this for you. You should keep overdrive off for now and have your transmission serviced. New filter & oil & flush the cooler. Make sure you ONLY USE THE RECOMMENDED FLUIDS!!!!!!!!!! See if that helps. Do not use overdrive pulling a trailer and going up a hill and do not use it if you are on windy roads where you are slowing down alot of times. Try letting off the gas for a second when you do want to engage the overdrive. Remember to always check your automatic tranny fluid with the engine running and fully warmed up.

I’m happy to assist further over the phone at https://www.6ya.com/expert/kim_3202917d7c587455

I am surprised the AAMCO techs have no ideas about this. This "slipping" could be caused by the overdrive system having a mechanical problem (e.g. sticking solenoid, leaking seal) when trying to engage or disengage.
The overdrive solenoid locks the torque converter when activated, and releases when deactivated, both starting to happen when the vehicle is above about 40 mph and depending on what the vehicle is doing at the time (accelerating, decelerating or coasting). When you depress the accelerator and the solenoid is engaged at close to this speed, it should immediately release, and then re-engage when you stop accelerating, and you should immediately feel a light "bump" each time, not a "fish bite".

When the OD light flashes in this truck it means a malfunction has occurred in the transmission, it usually will be due to excessive slippage of the transmission based on the signal of the input and output shaft speed sensors. Here is what you need to do is have the Transmission Control Module tested for the fault code or codes that was stored when the light starting flashing, it is doubtful the valve body is the problem based on my 30 years as a Ford dealer technician.

I had a car that did that and in the end my computer was not working properly. I replaced the computer with a used one out of another car and is ok now.

You might try replacing the thermostat. Hows the heater work? That will give you a clue. A stuck or defective thermostat will cause poor fuel mileage and poor heater performance. This vehicle needs a
192-195 degree thermostat to operate correctly.

one question at a time, the OD flashes to let u know that the transmission control module has detected a fault in the trans, this is useualy slipping, u may not even feel it. Have it tested for trouble codes.
As for the AC, is the system fully charged, does the compressor run?

the OD light is flashing to let u know a fault has occurred in the transmission electronic controls or that it is slipping plus other issues, go to Kragen or Autozone and ask them to run a trouble code test, it is free, post the code back here for me to look at.