Picture this, you driveline is 20" long. 4" is the u-joint by the transfercase, 6" is driveline tube and the remaining 10' is the combined slip yoke and bottom u-joint. When you droop the passenger side the driveline pulls apart since there is only 6" of extension available. So you buy a longer slip ass. from someone that has 8" of travel available, if you were to just cut off the old slip and weld the new one on the driveline would be 2' longer than it was. So to compensate for that you need to cut the driveline tube 2" shorter and weld the new slip on. Now your driveline is still 20" long but now it has 8" of slip.
