The amount of lowering does not have much impact the requirement of a new driveshaft or not. One can experience vibrations with the extended control arms at both stock ride height and at a lowered one. It really seems to depend upon the tolerance stack-up of each vehicle - some get a vibration right off the bat, others are OK up to a certain MPH (driveshaft RPM).
I had a set of 3/4" extended arms on a 2" lowered SS that was fine at regular cruising speeds, but did experience a vibe at higher speeds.
If you want to center the rear wheels, you must use BOTH extended upper and lower control arms. If you use only the lowers, it will screw up your pinion angle tremendously.
Mike - '94 BBB SS (RIP 06/16) - '95 DGGM SS - '96 DCM SS - '92 BBB Wagon