I called Tom Woods this morning to get their take on this. They brought up a few things I hadn't thought of.
Double cardan joints are longer than u-joints. On a SWB jeep, there just isn't enough driveshaft length to stuff 2 in there.
Double cardan joints are heavier than u-joints. You could end up with a drive shaft that's so heavy it's hard to balance. Not good for freeway driving.
There is a double cardan pinion yoke for Dana 44's though, so the flange wouldn't be necessary.
It's always good to talk to the experts.