I put a set of the Sulastic shackles on this weekend. Took me about 3 hrs by the time you drop the hitch, then the top "superior" shackle bolts didn't want to come out. I've had the dreaded 45-50mph hop ever since I bought the truck, and these shackles to seem to help alot. Most of the vibration is gone, and the ride is definately smoother. I'll let the wife drive it for a couple of days to see if she can tell the difference. Well worth the money so far.
'06 GMC Sierra 2500HD Duramax, Crew Cab SWB, BF Goodrich All-Terrain T/A KO's 265/75R16's, B&W Turnover Ball gooseneck hitch w/ 5th Wheel hitch, pretty much stock.