Steve,
This discussion won't be solved by just thinking about it.....
So, why don't you just go out to your van tonight with some spacers, a
jack, a ruler, spring compressor and a six pack.
Open a beer, take a swig.
Measure the compressed length of the spring.
Mark the rear shocks compressed as they are now, jack a wheel up so its
off the ground and mark its extended length.
The difference between the extended length and the compressed length is
the sag.
Take a swig.
Insert a spacer.
The extended length of the shock shouldn't change since it is topped
out.
Lower the van back down and again mark/measure the compressed length the
shock with the jack removed.
Measure the compressed length of the spring (just the spring) with the
spacer in place.
Take another swig.
Subtract the new compressed length from the old compressed length.
The difference in compressed lengths of the shock before and after
inserting the spacer should equal the thickness of the spacer and the
length of the weighted spring should be the same....If not then your
premis is right--drink remaining 5 beers.

i'm thirsty,
fred