Yup, it is a transmission issue, but, it isn’t what you might think. Your 2006 Expedition has a complex suite of electronics that run the transmission. For example, the engine control module (ECM) is the master computer of your engine. It not only records issues as they occur, much like a good digital tape recorder, but it also takes input from as many as 16 different sensors, control modules and the like. The ECM, on receiving all of its inputs, determines exactly how your transmission will set up and operate.
For instance, if the manifold air pressure (MAP) sensor sends information to the ECM suggesting that not only do the transmission settings need to be tightened but also suggesting that the fuel-air requirements for the fuel-injection system need to be tightened as well, then not only will the tranny operate more efficiently but the ignition system will do so as well. Think of it, the tranny helped to determine how the engine should work, as well as the transmission.
The same is true of the MAF or mass airflow sensor. Indeed, there are sensors at each wheel, watching the crankshaft and the transmission, as well as ignition system and more that keep your Expedition running correctly.
What I suspect is happening here is that that ECM is on its last legs and has to be replaced. At one time you would have to step through a whole range of diagnostics to figure out what is happening in the transmission because transmissions, at that time, were primarily mechanical and the control systems were mechanical. With today’s computer technology and the extensive use of sensors and CPUs to keep things working correctly, it is fairly easy to diagnose the area where a problem may be occurring.
That’s why I pointed to the ECM because of the reaction of the transmission. To figure out the exact sensor or control module that might be responsible, you still do have to go back to the basics and run a diagnostic scan and then go through the results printout to see where you should be checking. Let me know what happens, I’d be interested to learn.