1997 E4OD O/D Light flashing above 100 km/h (60 mp/h)

Hey I got a funny situation, not really stressed about it, just wondering what the steps are for diagnosis.

So I was driving the other day, truck just got 35's installed and it's fun to drive, so I've been punching it a lot more than usual.

Did so from a crawl to jump over to the HOV lane, and pushed the rig pretty hard, it kept the gear and red lined it, then was a rougher shift than usual. (Used to a 92 IDI E4OD with a HD Shift kit) Compared to that it was mild, but the O/D light started flashing. Truck drove normally so I got it home and turned it off. Light does not come back on when you start it, no rough shifts, however it will start flashing as soon as I hit 100 km/h (60 mp/h). Once it's on the truck feels maybe slightly sluggish, but nothing dramatic, no issues shifting, just doesn't feel 100%, more like 80-90%.

Here's the kicker, doesn't throw a CEL, or have a code that I can find with my reader. Checked fluid level and it is good, and the colour is fine.

Now I have a spare E4OD with like 180,000 km's on it that is mint which I could throw in, so I'm not worried about the cost of a rebuild, or freaking out. Just curious how to diagnose it, as if it's something simple, it'll save me a days labour.

Thinking it might have to do with the torque converter.

It'll do it's flashing thing between 80-100 km/h. Still no real power loss, or shifting issues that are noticable.

Yet if I turn the O/D off it'll run all the way up to 130 km/h (highest I've pushed it) without flashing, and not flash till I turn the O/D back on then it'll flash again.

Well turns out it's either the torque converter or the pump for anyone who might stumble upon it later.

Gonna get a junkyard E4OD (turns out the spare was the smaller one that goes with the 4.6L) rebuild it and install a shift kit.