Intermediate clutch piston seal was installed backwards. This is a lip seal, so it was probably leaking hydraulic pressure from day one (F5 pump may have helped mask the problem). 1-2 shift always did seem a touch weak but it never slipped so I didn't think much of it. I can't remember if I put the seal in, or if I simply failed to replace it when I first rebuilt the transmission. Either way, I should have caught it. Not the E4OD's fault.
Internal leakage at that piston is probably what took out the center support bearing nearby. Then that caused loss of pressure on most of the clutches due to the drum hub going out of alignment on the sealing rings and...that was about all it took. About an hour after I noticed it pulled weak from a stoplight, the transmission was barely able to limp home due to severe slipping. It would still move around the yard as long as it was cold. As it warmed up, it would start slipping again so I knew it was done.
I also found the overdrive return snap ring had popped out, but apparently it doesn't matter much unless you are using the later bonded piston style setup. I should have replaced it with a spiral spring 13 years ago.
All in all, I think it did well considering the assembly error. I sent the billet converter to get rebuilt just in case shrapnel was in it, but apparently it looked like new inside. I'll be replacing the cooler and then adding a magnetic filter on the return side just to be safe.
If it wasn't for my goofs, I have no doubt it would have kept going for many more years.