I've searched everywhere and haven't found anything similar to my transmission issue. I have a 97 F350 7.3 dually 2 wheel drive with a E4OD. For years my truck has been shifting from 3rd to 4th gear when I start going uphill or accelerating while driving at higher speeds, like the highway, and as soon as the truck is no longer under load it will downshift into 3rd. It's the opposite of what it should be. I'm rarely on the highway but always hauling a trailer just shy of 10,000 lbs and average 3-4,000 miles a year.
13 years ago I had the transmission replaced with a heavy duty one from Ford. For at least the past 6 plus years, the transmission has been shifting like that. 18 months ago I drove it an hour away to help a buddy with a job, mostly highway. When I got off the highway the transmission was hot and leaking, noticed by the cloud of smoke. I got to the job site just off the highway and left a puddle. It did the same thing on the way back. I added transmission fluid and it was good till about a month later when I drove on the highway again and after 6 miles I could see it smoking so I pulled off into a rest station and had someone bring me transmission fluid as it was pouring out and hot. When I started it up and put it in drive it was slipping and didn't make it back home. I had it towed to a local transmission shop and they rebuilt it. It shifted the same way when I got it back as it had been. At just over 900 miles, almost a year later, it wouldn't shift out of second but I slowly made it home. The next day I was going to drive it to the transmission shop but it wouldn't go into gear so I had it towed. They said the flex plate cracked and broke the transmission pump, but I don't believe them as there was no noise and it started without issue. A few weeks later it didn't want to shift out of 2nd. Drove it to the shop and they said debris binded up the valve so they flushed it after taking it apart. A few months later I got on the highway again and once I got out of the traffic the OD light started flashing and after 15 miles I exited and once again I saw smoke but made it to the job site a few miles away. Got out and it was leaking. After we completed the job I started the truck and it didn't want to into gear but after giving it some throttle it started to move. I had no issue driving 4 miles to get transmission fluid. I kept it in drive with the parking brake on with someone in the driver's seat and got one quart and put it in. I didn't check how low it was. I made it back home 15 miles through town without issue. I backed into the driveway and had someone disconnect the trailer as I didn't want to put it in park. I pulled forward and then backed up to dump what was in the bed and then drove it the mile to the transmission shop. The owner saw me pull up and walked over. I explained what happened and he had me pop the hood. When he pulled the dipstick it was hot but the fluid wasn't burnt. When I went to pick it up he said the torque converter overheated but didn't know why. The next day I drove about 2 miles to the job and it shifted fine. When we left it didn't want to shift out of 2nd till I let off the throttle. It did that the 2 miles to the 2nd job which was back towards my house and then to the transmission shop. He again said he pulled it apart and flushed it but found no cause for the issue. Over the past few weeks I've driven it about 40 miles in town and it's shifting like it was before he rebuilt the transmission but I haven't been on the highway yet. I believe if I can solve the down shifting while cruising down the highway it won't got hot and will hopefully take care of my transmission problems and give me better mpg when on the highway. Right now I don't trust driving it down the highway.
Does anyone have any experience with this type of shifting issue? Once again it will shift from 3rd into 4th when going uphill or accelerating and then back to 3rd when not under load. The RPM are high when it down shifts and it doesn't shift hard.
Thank you in advance......

 

The transmission will shift because the ECU will send a signal to make it shift. The ECU takes signals to determine what is going on. It needs to know how much throttle, speed, etc. I would check those sensors to determine which is not working correctly. I believe the diesel engines have something that is attached to the throttle linkage, but someone here will let you know. The speed signal comes from the speedometer [PSOM].

If I got the transmission rebuilt and there was a problem, I would not wait weeks/months/years to bring it back. I also would not leave the shop until I went on a road test to confirm your problem. Dropping off the keys seems to rarely get clients problems fixed right the first time. Leaking oil when it gets hot is a problem.

 

That means that codes are stored in the PCM. You need those codes to diagnose this.