Yes, you can build an FX4 L2 clone but only Ford can build an authentic L2. Although, I have an FX4 that is the Ford built equivalent of an L2 . The main things have been listed so far are the Torsen rear diff, Bilstein shocks, better skid plates, 31" BFG AT tires, unique stainless steel front tow hooks, unique Alcoa alloy wheels, and manual transfer case shift with manual transmission in 02 an 03.
A few minor details that have not been mentioned are the unique seats on earlier models, black rear tow hook (unless a factory receiver was installed), black valence and fender flares on earlier models, of course the FX4 L2 decals, the sticker on the inside of the gas flap that lists the 31" tires and pressures for them, and I think the interior vinyl was uniquely black to FX4 L2 or at least on earlier models (not sure on that). I am sure later models had other items unique to an L2, but I don't know about those.
I looked into switching to the manual locking hubs. You do have to switch a lot of stuff on the front end to the vacuum hubs that were used in 98, 99 & part of 2000. They use a smaller wheel bearings and are not as durable. You also have to buy the conversion locking hubs and many people had problems with those. I decided it was too much work and too expensive (for new parts) to end up with something that was less reliable. I spoke with a Ford engineer about the difference in mpg and was told it cost about 0.3% in gasoline. I drove on a long trip once with the front driveshaft out and I got better than a 0.3% increase in gas mileage but it was not much more than 0.5%. I swear it felt peppier though and I could not feel a very slight vibration that I normally get at high speeds. That happens approaching 90 mph and I don't go that fast very often. So far at almost 260K miles, the only thing on my front drive train that wore out was the CV joint boot. That let all the grease out and wore the CV joint before I discovered it. It ended up being easier to installing a new front driveshaft, and found a source that was good quality at a reasonable price
It was stated that you need unlocking hubs if you run a locker or LSD in front. I can imagine that may be true for a locker but I have been running a Torsen front diff for many years and do not feel any difference in 2WD than it did with an open front diff. There is a slight on center stick when I first drive it when the temperature is near 40 or lower. It is barely noticeable and I think many people would not even feel it.
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