I owned an '87 Cherokee Sport with the AX-15, and 200k on the factory fill. (purchased off friend who was original buyer). Trans became very hard to get into gear, especially when cold. The brass synchronizers were worn I suspected. I joined an XJ forum and read tons of threads about the transmission fluids and decided on the specified 75W-90 synthetic GL-4 oil. The fluid GL-5 supposedly was a no go in them, supposedly the friction modifiers were bad on the synchro's. The fluid didn't do anything for the forces shifting into gear, or cold performance.
I did much more research on the forums, and some were using motor oil in with positive effects. 10W-30, 40, and 50W oil. I filled it with 10w-40 synthetic and it was much better in every respect. Shifts required less force into gears, cold weather operation was very much improved, and I was shocked that it had that much effect on it.
The transmission lasted another 100k with rough use, towing heavy boats, car trailers, and some off road use also.
I'm not saying that fluid will "fix" a transmission that needs a rebuild. But in my case the fluid change decreased the forces needed to shift into gear. I still can't understand why the fluid had that effect on the synchro's, but apparently helped in my case.
Another fun fact, the jeep had its original clutch too, still performed well without any issues at all. Now, the hydraulics for the clutch on the other hand, thats a sore subject..