That's not correct. The 7" lsds on the 4x4 also have bolt on driveshafts. In fact all cars with rear disc brakes (that includes loads of normal Sierras) have bolt on driveshafts.

The difference between the 7" and 7.5" can obviously be seen on the size, but that can be difficult when you don't have them next to each other. An other thing to look for is that the 7" diff has 2 long bolts (one at the bottom, other at the top) going through the diff to mount it in the beam. The 7.5" diff has only one 1 long bolt at the bottom and uses 2 short bolts from both sides for the top mounting point.