I'm a little confused. In your first sentence you say you want to upgrade the axle of a 2003 FX4-II. That comes with a Torsen I believe. Or do you mean upgrade TO that axle, because you have a 2002 XLT in your sidebar.
The Torsen is a pretty good diff. If I found an FX4-II axle with it and it matched my front gears, I would swap that axle in. I would not buy a Torsen and install it--not a new one. Maybe not even a cheap used one.
Torsen and the clutch type axles are attempts to maximize traction and make it a smooth as possible so as to mask it from the driver. There is a compromise to both traction and smoothness. The clutch type is very simple and uses the natural side thrust of gears against each other to engage the axle without traction--and it does work pretty well. The Torsen uses inefficient planet gears to increase resistance to the act of differentiation, and they also increase their inefficiency through side loads, just like the clutches. They work pretty well. But they aren't as smooth on the road as a differential with efficient planet gears, and they aren't anywhere near as good when all hell is breaking loose as a real locker.
If I were going to replace a diff in a 4x4, I would replace it with a Detroit Locker. You don't need to mask the rear diff from the driver because you are the driver. If you know how the diff works, you can drive it smoothly. Any time, any conditions, you always have maximum power to either rear wheel. The axle is locked solidly--period. The only time a tire can unlock itself is if the ground pulls it along faster than the other tire. It only happens when you are going around a corner with traction--the outside tire has to take a longer path and speeds up, unlocking itself. It's very smooth and after the Detroit is worn it, you will forget about it. When you need it, both tires are digging 100%.
I've had a Detroit in my Chevy crewcab 2wd for 14 years and you would not be able to tell it was there, even if you were an 80 year-old granny. I've had a similar object in both ends of my 4x4 Ranger for as long. You will notice it if you floor it going around a corner and the inside tire spins, cathces up with the outside tire and then they both spin and the rear slips out from under you. That can happen on ice or on side-slopes on hills, so you need to be careful while you are getting used to it. But once you have conquored it--and it's not difficult to understand it once you drive it for 15 minutes--you won't regret it.