If the CEL came on w/o the RPMs changing, I don't off the top of my head know the cause. But if a rebuild solved it then it was one of just a few things, because we can eliminate the PCM as part of the problem.
* new solenoid, if they replaced it
* valve body cleanup may have resolved an intermittent issue
* new torque converter resolved an internal clutch issue on the old TC
All those things are easy to do yourself, but the TC involves dropping the transmission which is beyond what a lot of people are willing to do. The solenoid & valve body are also extremely easy even with the trans still in the Jeep, but they fall into that "black magic" category of automatic transmissions that many people are unwilling to dive into. I only know they're easy to do because in my old age I've become way too cheap to pay for someone else to do something to my Jeep. IMHO, the rebuild is not even close to worth $2k. But I guess I value my money more than my time.
it is definitely a breeze to rebuild, even for the truly incompetent, lol. If I could find cores for sale I would rebuild them on the side and pocket some nice side hustle money. But 32RH are like hens teeth. They're the first thing to go on wrecked jeeps and still fetch $2k price tags. What I really ought to do is figure out what old Mopars used this transmission, and then figure out what changes need to be made to up-fit them into a TJ.
I had a C4 trans rebuilt about 15 years ago by a reputable shop, and I swear it was a lifetime warranty, provided I came back in for routine checkups. I never did that, but in hindsight I'm pretty sure those routine checkups were going to involve fluid flush, new filter, etc and would end up costing me a ton of money in the long run. Also in hindsight I think I got railroaded into an unnecessary rebuild. It was only $1200 iirc. I'm still kinda irritated about it.