Tom: There are actually a few different double cardan joints out there. The early ones were the SMALL pattern and are actually not as strong as the 84-85 double cardans. There is a size difference in the center bearing. Sure it isn't a marshmallow, but there is a significant difference in strength. People have been known to break the early ones, but the late ones are really tough to break.
The IFS double cardans have hardly any flex to 'em unless you take the time to take them all apart and grind the crap out of 'em for clearance. Too much work for me. Hold out for the big pattern solid axle double cardan.
Interestingly, on the single u-joint end, the IFS truck's front driveshaft can handle more angle than the front single u-joint on the solid axle shaft (without grinding).
So the ultimate is a 1984-1985 front double cardan and a 1986+ front U-joint.
*shrugs* lots of other combos work just fine out there in the rocks too.