So, everyone who said the stock trans in these trucks can barely handle the stock power at best weren't kidding. A few month of towing with the fuel plate FF and the TC started slipping while in lockup pulling heavy up steep hills. Only did it a few times, but still. Once is all it takes for things to start going south.
I pulled trailers all morning the other day, it was doing fine and no sweat when I got off the highway and a few miles up a 50mph road when all the sudden it started dropping out of OD. Usually this hunting has been a TPS problem, so I pulled my meter out and checked it. All within spec, all cables hooked up, everything as it should be. I continued on my way, with no OD at all now. Which is to say, I had no TC lockup in or out of OD. A few miles farther up the road and the TC started letting go completely while leaving a stop or climbing any grade at all. I dropped the trailer and had someone come get it, and limped back to the shop with a top speed of 50mph. Aside from no lockup, it didn't give me any issues without the trailer.

So, I'm now faced with what I believe is a TC that is in the process of grenading in a tranny that I've hated since it was put in. I wish I had time to have one built or get the parts to fortify it, but I don't. I need a drop in replacement now. There's one option in that regard which I've been considering for a few months leading up to this: we have an excellent relationship with TransAxle LLC, who gives us a good fleet discount and can have a tranny to me next day. I can't find their performance builds on the website, but I've talked to them extensively and what they're recommending me is their stage 2 build, with a billet single disc, hogged out VB, upgraded servos and some other stuff I don't remember rated for 550HP, which I'll never get close to. 3yr/100,000mi warranty. My fleet cost is $2500. I'm 99% sure I'm going with this, as if I call them Monday I'll have it Tuesday and Wednesday I'll be going again.

My questions are, does it make sense while I'm in there to do a billet flexplate? At what power/abuse level does it become warranted to have one? And more importantly, how concerned should I be about the fluid in the cooling system? Should I try to flush it all out? I'm assuming yes, but I'm also assuming to see what's in there now full of disintegrating TC. If it looks good though, what are the odds that it is?