I wouldn't be so sure about it being a transmission problem, at least without additional data.
My suggestion would be to set up a dashboard on one of the OBDII apps (I use Torque Pro since I've had it a while) and monitor to see what the PCM is commanding the transmission to do. If the downshifting is being commanded by the PCM, then the transmission is only doing what it's being commanded to do. If it downshifts without a corresponding command from the PCM, then the transmission has some sort of issue.
I'd also monitor the PCM's status for the TOW/HAUL switch. It may be have the mode engaged without your knowing or intending it.
Good morning... this seems like a very knowledgeable analysis of the problem. With my longtime (60 yr), albeit limited, knowledge of automotive engines and transmissions, I seriously doubt this is a transmission problem since the tranny functions perfectly in every other way, driving thousands of miles over the course of weeks on end on some of our more recent trips. The rig only has about 21,000 miles on it and drives like new... and btw this behavior has been present ever since the rig was new. I just have tolerated it and never done anything about it because it is difficult to find a qualified tech in a place where this behavior can be demonstrated.
Your suggestion that "the downshifting is being commanded by the PCM" makes a lot of sense, whether that might be some internal command or it may well be that PCM may be engaging the TOW/HAUL switch surreptitiously without some visible status indicator. I can tell you that if this *is* the issue, it is definitely not lighting up the dash indicator which always shows up when intentionally engaging the TOW/HAUL switch which I do use sometimes, but not always since the same kind of downshifting occurs whether switching it on or not. And it always has. So that lends some credence that the TOW/HAUL switch is being commanded by the PCM without my intentionally turning it on. It may be worth noting that the tranny will drop to a lower gear on its own whether in "D" (drive mode) or manually dropping it one gear. Whereas I would be happy to have it use engine braking in 5th or 4th gear (I think it would be), it will still drop down further into 3rd just like the TOW/HAUL mode would have it do). And when it does this, I fell obligated to tap the brakes occasionally anyway to keep the engine RPMs in a moderate range (say below 4000) both for noise mitigation as well as engine wear and tear.
I presume "PCM" stands for "Power Control Module" or something to that effect? When you refer to "one of the OBDII apps" could you make a specific recommendation?? If I can make a valid diagnosis it may help me guide the repair center directly to what needs to be done, since I don't have the expertise to make such adjustments or repairs on my own, I'm pretty sure. The factory repair center in the Portland area is Northside Ford and that part of town is pretty level. To demonstrate the issue to a technician we would need to drive up towards Mt Hood and turn around and head back down to encounter the kind of grade needed to evoke this kind of behavior. It is never a problem driving around in more level areas... it only occurs on steep downward grades and it's going to be very difficult to have a tech onboard for the length of time it would take to demonstrate the behavior. Hence some detail from an OBDII app done on my own time might help the tech effect an adjustment or repair.
I'm going to describe one other anomaly which may or may not be related. The speedometer on this rig has never corresponded directly to actual speed. When my GPS says I am doing 65mph, the speedo with read about 74... like wise when it reads 65 I am only doing about 56 or 57. I have always thought the speedo was simply never calibrated for the tire size properly and that's all there is to it, but now I wonder if maybe it might be related to the automatic downshifting as well?
Again, thanks for the indepth suggestions and any followup you can provide and then I'll take it from there, although probably not this time of year. It would need to wait until spring now.