FYI, the term "CV" is used in two different ways on driveshafts. One way is an explanation of what the joint does -- and in that case, there are several different types of joints that can rightly be called CV (constant velocity), including the double cardan, the joint actually named a CV (has balls in sockets) and other rubber-type/rag joint type joints like the late Explorers used.
The other use of CV is for one particular joint -- typically the one that drives the axles on front-wheel drive applications, with big ball bearings inside a collar that allow for flexability in a rotating joint.
A double cardan joint is a doubled u-joint setup, where two u-joints are connected by a ball and socket into a single unit that allows for a constant velocity rotation, in that one joint cancels out the rotational forces of the other while they turn (one joint is at 20 degrees, the other at -20 degrees, and vice versa = a smooth running joint without the hitch in rotational force required to transfer torque from one leg of a cross to the next).
Best way to fix an Explorer front driveshaft is to find another better one in the pull-a-part. Otherwise, find a local driveshaft rebuilder. Almost every city has at least one somewhere. Cost should be in the $125 range, typically. For heavier duty parts, check out Tom Woods, High Angle Driveline, etc., all of whom build some awesome custom driveshafts.
About the weak design of the CV driveshaft... It works. A lot of guys want to really ramp up driveshaft strength, and they end up twisting off something else, like axle shafts. I'd really rather loose a shaft or easy to replace u-joint than something more expensive in the axle.
