Ya, it is all dependent on your lift you buy. Each truck will be a little different. Some may need a slight tcase drop some may not.
The lift companies also can only generalize what your setup may be too. They can only account for stock trucks which as you know most of us don't run. That is why I say you may need to shim it and or drop the tcase.
There were also alot of different trans/tcase combo's available throughout the years. The lift companies have to generalize and ball park things because of the different drivetrain lengths. Also you have to think the same springs are used for K5's, K10's (short and longbed), and K20's. Each driveshaft is totally different length and may need different shims or drop amounts.
Harley