My 72 has a 4 inch tough country lift (all springs). I had to have the rear drive shaft lengthened. When I put it all together I was getting drive line vibes, so I took some angle measurements and ordered a set of 0 rate blocks milled with the angle I needed so the pinion would match the transfercase. The wedge shape blocks also gave me another inch in the rear to fix that saggy butt problem. And, my vibes are gone.
For 4 inch and even 6 inch, I don't think you need a double cardan shaft. Look at this site:
ColoradoK5. They are the true experts at lifting K5s.