Generally on a short arm your driveshaft gets shorter as you lift. The control arms move in a tighter arc then the driveline.

 

So longer driveshafts are not needed? How about for a 1" stretch?

 

It depends on how much spline you have in the driveshaft but even with a 1" stretch it is likely you don't need to lengthen the shaft. Most people lift and run longer travel shocks and that gained travel is generally on the droop which is where the driveshaft becomes shorter. When you compress the suspension and the control arms become level is where your driveshaft will be at its longes length and people almost never fully compress both rear wheels at the same time.