Seems like more issues on the horizon.

So on top of the issue with my U-joints going out, I am starting to have this issue and I can’t figure out if it’s my torque converter going out or my transmission. A little side note, a couple weeks ago a belt shredded off and put a huge gash in my transmission cooler hose from the tranny to the rad. Got it fixed that day after a few hours of dealing with headache, since I filled the tranny with the proper amount of atf, it shifts smoothly now and holds its gears… until today. I was driving out of town and it still shifts smoothly from first to second, but when it hits 35mph to 40mph it can’t decide whether it wants to be in second or third and when I give it a little more pedal sometimes it keeps it from doing that, also sometimes it will do it, and sometimes it won’t. Once I hit 45mph no issue, drives just fine. Can anyone think of the cause for this issue?

Sounds like a vacuum regulator issue... although since you have an IDI, I don't know how they're accomplishing such?

Has it always done this since the 7.3 swap, or is this a new issue?

It’s a new issue, I have seen a long time ago a person was reproducing aftermarket vrv’s. I believe they still are but I want to check if this one is still good. I know you can adjust it but I need to get a new vacuum tester. Mine was harbor junk.

If you haven’t, I would double check the tranny fluid level.

And pull the hose off the modulator to see if you have fluid there, shouldn’t be any, when mine went bad it leaked through the modulator.

That honestly could be the issue. This motor has leaked from day one of owning it. A lot of oil and diesel and atf is all over the engine and engine bay so maybe there is fluid in there but I did check the level this morning and is in a good range.