For what year GM? They changed to longer rear springs in '88.
I have a pair of 9-10" lift Atlas rear springs brand new, never used. They fit 73-87 1/2 and 3/4ton pickups with 52" rear springs and 73-99 Blazer/Tahoe/etc.
I also have a rear driveshaft, but it has 1410 u-joints at both ends. The shaft can be modified, or you can upgrade your t-case and pinion yokes.