I have been following a GMC board for awhile because they have the same problem with freeway bounce. A guy finally installed the Sulastic rubber spring shackles and here is what he said ....
"Here is my report. In a nutshell they did help...
The truck is still very stiff and the bounce is still somewhat there. It has dampened the bounce quite a bit not did not remove it completely. I would say that it removed about 50%-75% of the bounce and at higher speeds it improved the handling a lot more.
The truck still feels like it would benefit from more weight in the back. On speed bumps the truck still rides stiff when you go over them slow but go over it fast and it absorbs it very well, much better than before. On a rough road it smooths it out, I would not use the word silky but more like the handling of the front suspension.
The bumps on the freeway were the biggest concern and it did not smooth them out as much as I had hoped for, the truck is still very stiff and may need to be worked in a bit more but I don't know.
Was it worth the $400 plus labor? I would say depends on if you can afford it, if it doesn't bug you then don't spend it, if you can't stand the bouncing then go for it, it will help some.
Lowering the truck does look much nicer and set the weight back a little but again it feels like a light rear end when driving. May put a few cinder blocks in tonight and try again.
I was hoping for better but at this point I will not return it because it does improve the ride all around and for $400 having a better ride all around is well worth it to me as I plan to drive this thing until it dies. "
here's the link if you want to read more:
Sierra Bounce - GM-Trucks.com
my bounce has been greatly softened by adding my shell and breaking in a little, though i'm considering Sulastic since that with my shell may make it a Cadillac without losing any payload.