The sway bar drop brackets move the U shaped brackets on the crossmember down 3 inches or 1.5 depending on which kit you buy. Since the axle is farther away from the frame with the lift, the sway bar turns downward more. When you suspension articulates, it puts more stress on the cross member and the U bolt brackets. Think of the sway bar as brake lines and what happens to them if you dont extend them. Obviously you cant extend a sway bar, but you can change the mounting point.
As for the drive shaft, I assume you have a CV shaft. Buy getting a longer front drive shaft, all you are doing is making sure it doesnt come apart at the slip. The spacer is going to move the pivot point of the CV joint forward an inch so it clears the cross member.