I would bet on some kind of debris got the boot, if it was replaced before with the CV joint. I've only had one boot tear, and it was likely the original on my last 98.
I plan to upgrade too someday, but the choices are very few from what I've seen. Nobody makes anything special for these trucks. The old 4WD swap project used an earlier Explorer front shaft without the CV joint, but some have reported vibrations. I think the joint angles are a good question to research before making a new design shaft. I will also want a stronger shaft, and the later trucks all used double CV joints with 8.8 front diff's. So it is a tough subject, to upgrade the front shaft. I'm not sure what the best path will be.
For yours I'd stick with a stock type, it's just fine for anything up to moderate power and rpm's, which is where you are now. If you will push 500hp or more, then more strength would be a good upgrade.
Here's one other thing to watch, keep an eye on the differential bushings, the rear one specifically, and the frame mount for that. Watch for any excess looseness of that bushing, the bolt, or cracking of the frame bracket. My bracket there cracked and eventually broke loose. I think the large forces at launch are high enough that it might eventually break the welds of that bracket. Mine did that, the mileage was about 260k then. I think that could be why the last versions of the chassis(late Rangers and Sport Trac), they have a longer diff/bracket so they mount farther back on the frame. I plan to upgrade to that bracket on my project truck.
