I was browsing the 4wd.com website when I saw that they now have ProComp front and rear driveshafts available for JK's with 0-4" lift. And from looking at the pictures, it seems they are not a double cardan style like other shafts are.
This is from their website: The factory driveshaft ball and cage design joint is replaced with #1310 style u-joints to allow for a stronger engagement, increase the range of motion, and improve seal and driveline life. Exclusive yokes, not flange adapters, are used to allow for a longer driveshaft, unlike our competitors versions. Pro Comp Suspension rear driveshafts are constructed from 3.00x 0.095 DOM (drawn-over-mandrel) steel. Kits come complete with necessary mounting hardware.
Now I know ProComp isn't the most popular brand out their, but it seems that a single cardan is safer for the drivetrain then a double cardan?
2008 Flame Red 4dr, 4" Pro Comp, 37" Pro Comp Xtreme A/T's, 17" Pro Comp 7032's, G2 5.13 gears and covers, Pro Comp driveshafts, Smittybilt XRC f/r bumpers, XRC side armor, XRC8 w/ masterpull rope and fairlead, Rigid Industries Dually Floods and 10" spot bar, Trucklite LED headlights.
This will only work if the front pinon yoke and transfer case front drive shaft yoke are at exactly the same angle. A double cardan is used where there is an extreme angle, or where the angles aren't the same, both of which are likely if you lift the vehicle more than 2.5". Not sure what your situation is, but bear in mind excessive driveline vibration on JKs is turning into more than just an annoyance. Cases are showing up where transfer cases on Rubis are literally exploding or shearing off the transmissions. Gotta find a way to smooth things out.