Unfortunately, I've not seen dissipation numbers for trans coolers like you can get for some radiators. It would be interesting if you could put a temp sensor inline with it. I know the old guys used to talk about heating up the transmission faster as a good thing as well as cooling it. Not sure if that's still valid with today's designs, or if it's just a northern thing. I never saw anything in writing about that. The most compelling argument was in the case of starting, especially cold in cold weather, and letting it idle to warm up. You get very little heat generated in the trans (it's just pushing fluid through the lines) but the engine heats up to it's temp of 195 or whatever. The trans only really contacts the block through the bell housing (it's only good heat transfer contact that is) and it isn't very efficient. So, the tail might be at 20 or 30 or 40F (outside temp) and the bell at 190F. Aluminum has a big coefficient of expansion, and it is made of several sections that are bolted together. This could make for big stresses as one part expands much faster/longer than others, especially at the bell housing, leading to cracking.
I always run all my engines at 160. The 4.0 CAN run cooler and mine gets to 180 in stop-and-go traffic with AC on in 98F ambient temps. That's all. Rubber parts don't last as well, and neither does a trans at higher temps. I used to have a trans temp on the return line of my Explorer. It would run at approx engine temp until a faster acceleration or accelerating while towing, then it would spike up to 30F higher, after the coolers, then come back down. That's all from heat made in the TC, so there's a lot. I also had a separate fan just for the cooler. You may want to do that too. Air flow is everything, and thickness reduces flow, so stacking another thickness in front of an already marginal design may not be well. I'd maybe put it on the electric fan side, with an add-on fan as pusher in front. That would help the current elec fan too. The other fan is much higher capacity to begin with.
If you want a cheap way to test temp, get a couple of thermistors (Radio Shack or other elec shops) and just hook them to a resistance meter (VOM, DVM, etc). Tape it to the input and outlet of the cooler (metal tube part), keeping the leads insulated. It's not very impressive for day-to-day looking cool, but you can get a reading on temp and convert the resistance to temp from a chart, or just remember what's a bad number and watch for that... You can unhook the meter when you're done, but leave the thermistors if you ever want to spot check later. Run some thin wires into the truck to read on the road (even out the hood and in through the door jamb).
Report back if you get to test it. I'd run some sort of gauge either way if you go solo cooler.
On the fluid leak, I know some Fords will cause the fluid to foam up if slightly overfilled. This, more than expansion, causes blow out on them. That also means the fluid can foam (also causes other problems when it does that). Maybe your leak is letting air in and causing foam too.
The other more obvious possibility is that if your cooler was faulty, and you had to take it out of the loop because it didn't have much or any flow, your fluid was getting REALLY hot from no flow! It'll keep expanding while it's still liquid. I reread your post and it said Jeep said to remove the valve, not bypass the cooler. You may want to keep that as an option after monitoring your temps.
Hope that helps.